Cambridge IAL · Thinka-original Practice Paper

2025 Cambridge IAL Mathematics (9709) Practice Paper with Answers

Thinka Jun 2025 (V5) Cambridge International A Level-Style Mock — Mathematics (9709)

350 marks520 mins2025
An original Thinka practice paper modelled on the structure and difficulty of the Jun 2025 (V5) Cambridge International A Level Mathematics (9709) paper. Not affiliated with or reproduced from Cambridge.

Pure Mathematics 1 (Paper 15)

Answer all questions. Show all necessary working clearly.
10 Question · 75.5 marks
Question 1 · Short-answer structured questions
4.5 marks
The coordinates of two points \(A\) and \(B\) are \((2, -3)\) and \((6, 5)\) respectively. Find the equation of the perpendicular bisector of the line segment \(AB\), giving your answer in the form \(ax + by = c\), where \(a\), \(b\) and \(c\) are integers with no common factors and \(a > 0\).
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Worked solution

First, we find the midpoint, \(M\), of the line segment \(AB\): \(M = \left(\frac{2+6}{2}, \frac{-3+5}{2}\right) = (4, 1)\). Next, we find the gradient of the line \(AB\): \(m_{AB} = \frac{5 - (-3)}{6 - 2} = \frac{8}{4} = 2\). Since the bisector is perpendicular to \(AB\), its gradient, \(m_{\perp}\), is: \(m_{\perp} = -\frac{1}{2}\). The equation of the perpendicular bisector is: \(y - 1 = -\frac{1}{2}(x - 4)\). Multiply both sides by 2: \(2y - 2 = -x + 4\). Rearranging into the form \(ax + by = c\) yields: \(x + 2y = 6\).

Marking scheme

M1: For finding the correct midpoint of \(AB\) at \( (4, 1) \). M1: For finding the gradient of \(AB\), which is 2. A0.5: For stating the gradient of the perpendicular line is \(-\frac{1}{2}\). M1: For attempting to form the equation of a line using their midpoint and their perpendicular gradient. A1: For the correct simplified equation \(x + 2y = 6\).
Question 2 · Short-answer structured questions
4.5 marks
A car of mass \(1200\text{ kg}\) travels up a hill inclined at an angle \(\theta\) to the horizontal, where \(\sin\theta = 0.05\). The resistance to motion is a constant force of \(300\text{ N}\). Given that the car travels at a constant speed of \(15\text{ m s}^{-1}\), find the power of the car's engine, giving your answer in watts. (Take \(g = 10\text{ m s}^{-2}\).)
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Worked solution

Since the car is traveling at a constant speed, the acceleration is zero, meaning the driving force \(F\) is equal to the total resistance forces acting down the slope. The component of the car's weight acting down the slope is: \(W_{\text{slope}} = mg \sin\theta = 1200 \times 10 \times 0.05 = 600\text{ N}\). The total resistance down the slope is the sum of the weight component and the resistance force: \(F = 600 + 300 = 900\text{ N}\). The power \(P\) of the engine is given by the formula: \(P = Fv\) where \(v = 15\text{ m s}^{-1}\). Thus, \(P = 900 \times 15 = 13500\text{ W}\).

Marking scheme

M1: For calculating the component of the weight down the slope using \(mg\sin\theta\). A0.5: For getting \(600\text{ N}\). M1: For setting up the equation of motion along the slope \(F = mg\sin\theta + R\) to find the driving force. M1: For using the formula \(P = Fv\). A1: For obtaining the correct power of \(13500\text{ W}\).
Question 3 · Short-answer structured questions
4.5 marks
A particle \(P\) moves in a straight line. At time \(t\text{ seconds}\) after passing through a fixed point \(O\), its velocity \(v\text{ m s}^{-1}\) is given by \(v = 3t^2 - 12t + 9\) for \(0 \le t \le 3\). Find the total distance traveled by \(P\) in the interval \(0 \le t \le 3\).
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Worked solution

To find the total distance traveled, we must first find if and when the velocity changes sign in the interval \(0 \le t \le 3\). Set \(v = 0\): \(3t^2 - 12t + 9 = 0 \implies 3(t - 1)(t - 3) = 0\). Thus, the velocity changes sign at \(t = 1\). We integrate the velocity function to find the displacement function \(s(t)\) (taking \(s(0) = 0\)): \(s(t) = \int (3t^2 - 12t + 9) \, dt = t^3 - 6t^2 + 9t\). Evaluate the displacement at the key times: At \(t = 0\): \(s(0) = 0\). At \(t = 1\): \(s(1) = 1^3 - 6(1)^2 + 9(1) = 4\). At \(t = 3\): \(s(3) = 3^3 - 6(3)^2 + 9(3) = 0\). The distance traveled between \(t = 0\) and \(t = 1\) is \(|s(1) - s(0)| = |4 - 0| = 4\text{ m}\). The distance traveled between \(t = 1\) and \(t = 3\) is \(|s(3) - s(1)| = |0 - 4| = 4\text{ m}\). The total distance traveled is \(4 + 4 = 8\text{ m}\).

Marking scheme

M1: For setting \(v = 0\) and solving for \(t\) to find the turning point at \(t = 1\). M1: For integrating \(v\) to obtain the form \(t^3 - 6t^2 + 9t\). A0.5: For the correct integrated expression. M1: For evaluating the displacement at the limits and realizing the need to sum the absolute values of the two segments of motion. A1: For obtaining the correct total distance of \(8\).
Question 4 · Short-answer structured questions
4.5 marks
The equation of a curve is \(y = x^2 - kx + 5\) and the equation of a line is \(y = 2x + 1\), where \(k\) is a constant. Find the set of values of \(k\) for which the line and the curve do not intersect.
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Worked solution

To find the points of intersection, we equate the two equations: \(x^2 - kx + 5 = 2x + 1\). Rearranging this into standard quadratic form: \(x^2 - (k + 2)x + 4 = 0\). The line and the curve do not intersect if this quadratic equation has no real roots. This occurs when the discriminant is less than zero (\(b^2 - 4ac < 0\)): \((- (k + 2))^2 - 4(1)(4) < 0 \implies k^2 + 4k + 4 - 16 < 0 \implies k^2 + 4k - 12 < 0\). We factorize the quadratic expression: \((k + 6)(k - 2) < 0\). The critical values are \(k = -6\) and \(k = 2\). Since we require the expression to be less than zero, the set of values of \(k\) is \(-6 < k < 2\).

Marking scheme

M1: For equating the two equations and collecting terms on one side to form a quadratic in \(x\). A0.5: For finding the correct coefficients \(a = 1, b = -(k+2), c = 4\). M1: For calculating the discriminant and setting up the inequality \(b^2 - 4ac < 0\). M1: For finding the correct critical values \(k = -6\) and \(k = 2\). A1: For writing the correct inequality \(-6 < k < 2\).
Question 5 · Short-answer structured questions
4.5 marks
In a geometric progression, the second term is \(12\) and the sum to infinity is \(50\). Given that the first term, \(a\), is greater than \(25\), find the value of \(a\) and the common ratio, \(r\).
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Worked solution

The second term of a geometric progression is given by: \(ar = 12 \implies a = \frac{12}{r}\). The sum to infinity is given by: \(S_{\infty} = \frac{a}{1-r} = 50\). Substitute \(a = \frac{12}{r}\) into the sum to infinity equation: \(\frac{12/r}{1-r} = 50 \implies 12 = 50r(1-r) \implies 50r^2 - 50r + 12 = 0\). Divide the quadratic equation by 2: \(25r^2 - 25r + 6 = 0\). Factorizing the quadratic: \((5r - 2)(5r - 3) = 0\). This gives two possible values for \(r\): \(r = 0.4\) or \(r = 0.6\). If \(r = 0.4\), then \(a = \frac{12}{0.4} = 30\). If \(r = 0.6\), then \(a = \frac{12}{0.6} = 20\). Since we are given that the first term \(a > 25\), we choose \(a = 30\) and \(r = 0.4\).

Marking scheme

M1: For using the formula for the \(n\)-th term of a GP to write \(ar = 12\). M1: For using the formula for the sum to infinity to write \(\frac{a}{1-r} = 50\). M1: For eliminating one variable to form a quadratic equation. A0.5: For obtaining a correct quadratic equation, such as \(25r^2 - 25r + 6 = 0\). A1: For solving and selecting the correct pair of values based on \(a > 25\) to get \(a = 30\) and \(r = 0.4\).
Question 6 · Structured
8 marks
A circle has equation \((x - 3)^2 + (y - 2)^2 = r^2\), where \(r\) is a positive constant. The line \(L\) has equation \(y = 2x - 1\).

(a) Find the coordinates of the point on \(L\) that is closest to the center of the circle, and show that the perpendicular distance from the center of the circle to \(L\) is \(\frac{3}{\sqrt{5}}\). [4]

(b) Given that the line \(L\) intersects the circle at two points \(P\) and \(Q\) such that the length of the chord \(PQ\) is \(2\sqrt{5}\), find the exact value of \(r\). [4]
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Worked solution

(a) The center of the circle is \(C(3, 2)\).

The line \(L\) is \(y = 2x - 1\), which has gradient \(m_1 = 2\).

The line perpendicular to \(L\) that passes through the center \(C(3,2)\) has gradient \(m_2 = -\frac{1}{2}\).

The equation of this perpendicular line is:
\(y - 2 = -\frac{1}{2}(x - 3) \implies 2y - 4 = -x + 3 \implies x + 2y = 7\).

To find the closest point (the point of intersection of these two lines), we solve simultaneously:
\(y = 2x - 1\)
\(x + 2y = 7\)

Substitute \(y\) into the second equation:
\(x + 2(2x - 1) = 7 \implies 5x - 2 = 7 \implies 5x = 9 \implies x = 1.8\) (or \(\frac{9}{5}\)).

Substitute \(x = 1.8\) back to find \(y\):
\(y = 2(1.8) - 1 = 2.6\) (or \(\frac{13}{5}\)).

Thus, the coordinates of the closest point, \(M\), are \((1.8, 2.6)\) or \(\left(\frac{9}{5}, \frac{13}{5}\right)\).

The perpendicular distance from \(C(3,2)\) to \(L\) is the distance between \(C(3,2)\) and \(M\left(\frac{9}{5}, \frac{13}{5}\right)\):
\(\text{Distance} = \sqrt{\left(3 - \frac{9}{5}\right)^2 + \left(2 - \frac{13}{5}\right)^2} = \sqrt{\left(\frac{6}{5}\right)^2 + \left(-\frac{3}{5}\right)^2} = \sqrt{\frac{36}{25} + \frac{9}{25}} = \sqrt{\frac{45}{25}} = \frac{3\sqrt{5}}{5} = \frac{3}{\sqrt{5}}\). (Shown)

(b) Let \(M\) be the midpoint of the chord \(PQ\), which is also the point on \(L\) closest to \(C\).

Thus, the distance \(CM = \frac{3}{\sqrt{5}}\).

The length of the chord \(PQ\) is \(2\sqrt{5}\), so the length of \(PM\) (half of the chord) is:
\(PM = \frac{1}{2} \times 2\sqrt{5} = \sqrt{5}\).

Since \(\triangle CMP\) is a right-angled triangle at \(M\), we apply Pythagoras' theorem:
\(CP^2 = CM^2 + PM^2\)

Since \(P\) lies on the circle, \(CP = r\):
\(r^2 = \left(\frac{3}{\sqrt{5}}\right)^2 + (\sqrt{5})^2 = \frac{9}{5} + 5 = 1.8 + 5 = 6.8 = \frac{34}{5}\).

Since \(r > 0\), the exact value of \(r\) is \(\sqrt{\frac{34}{5}}\) (or \(\frac{\sqrt{170}}{5}\)).

Marking scheme

(a)
M1: For finding the gradient of the perpendicular line (\(-\frac{1}{2}\)) and attempting its equation.
A1: Correct equation of the perpendicular line, e.g., \(x + 2y = 7\).
M1: For attempting to solve the simultaneous equations to find the coordinates of \(M\).
A1: Correct coordinates \((1.8, 2.6)\) or equivalent, and showing the distance is \(\frac{3}{\sqrt{5}}\).

(b)
M1: Realising that the midpoint of the chord is \(M\) and that \(PM = \sqrt{5}\).
M1: Applying Pythagoras' theorem \(r^2 = CM^2 + PM^2\).
A1: Substituting correct values into the formula: \(r^2 = \frac{9}{5} + 5\).
A1: Correct exact value of \(r = \sqrt{\frac{34}{5}}\) (or equivalent exact expression).
Question 7 · Structured
8 marks
A curve has the equation \(y = 4x^{1/2} - x\) for \(x \ge 0\).

(a) Find the coordinates of the stationary point on the curve, and determine its nature. [4]

(b) Find the exact area of the region enclosed by the curve and the positive \(x\)-axis. [4]
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Worked solution

(a) To find the stationary point, we differentiate \(y\) with respect to \(x\):
\(\frac{dy}{dx} = 4 \left(\frac{1}{2}x^{-1/2}\right) - 1 = 2x^{-1/2} - 1\).

At a stationary point, \(\frac{dy}{dx} = 0\):
\(2x^{-1/2} - 1 = 0 \implies \frac{2}{\sqrt{x}} = 1 \implies \sqrt{x} = 2 \implies x = 4\).

Substitute \(x = 4\) back into the curve equation:
\(y = 4(4)^{1/2} - 4 = 4(2) - 4 = 4\).

So the stationary point is \((4, 4)\).

To determine its nature, find the second derivative:
\(\frac{d^2y}{dx^2} = \frac{d}{dx}(2x^{-1/2} - 1) = -x^{-3/2}\).

Substitute \(x = 4\):
\(\frac{d^2y}{dx^2} = -(4)^{-3/2} = -\frac{1}{8}\).

Since \(\frac{d^2y}{dx^2} < 0\), the stationary point \((4, 4)\) is a local maximum.

(b) To find the limits of the region enclosed by the curve and the positive \(x\)-axis, we set \(y = 0\):
\(4\sqrt{x} - x = 0 \implies \sqrt{x}(4 - \sqrt{x}) = 0\).

This gives \(x = 0\) and \(x = 16\).

The area \(A\) is given by:
\(A = \int_{0}^{16} (4x^{1/2} - x) \, dx\).

Integrating each term:
\(A = \left[ \frac{4x^{3/2}}{3/2} - \frac{x^2}{2} \right]_{0}^{16} = \left[ \frac{8}{3}x^{3/2} - \frac{1}{2}x^2 \right]_{0}^{16}\).

Evaluate at the upper limit \(x = 16\):
\(\frac{8}{3}(16)^{3/2} - \frac{1}{2}(16)^2 = \frac{8}{3}(64) - 128 = \frac{512}{3} - 128 = \frac{128}{3}\).

Evaluate at the lower limit \(x = 0\):
\(0\).

So the exact area of the region is \(\frac{128}{3}\) (or \(42\frac{2}{3}\)).

Marking scheme

(a)
M1: For differentiating to get \(\frac{dy}{dx} = 2x^{-1/2} - 1\) (at least one term correct).
A1: Finding \(x = 4\) and \(y = 4\).
M1: Finding the second derivative \(\frac{d^2y}{dx^2} = -x^{-3/2}\) and evaluating at \(x = 4\).
A1: Correctly identifying the stationary point \((4, 4)\) as a maximum with valid reasoning.

(b)
M1: For setting \(y = 0\) and finding the correct limits \(x = 0\) and \(x = 16\).
M1: For integrating to obtain \(\frac{8}{3}x^{3/2} - \frac{1}{2}x^2\) (allow one coefficient error).
A1: Correct integration.
A1: Substituting limits correctly to get \(\frac{128}{3}\) or \(42\frac{2}{3}\).
Question 8 · Structured
8 marks
The function \(f\) is defined by \(f(x) = p - q \cos(2x)\) for \(0 \le x \le \pi\), where \(p\) and \(q\) are positive constants. Given that the maximum value of \(f(x)\) is 9 and the minimum value of \(f(x)\) is 3:

(a) Find the values of \(p\) and \(q\). [3]

(b) Find the set of values of \(x\) for which \(f(x) \ge 7.5\), giving your answers in terms of \(\pi\). [5]
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Worked solution

(a) Since the range of \(\cos(2x)\) is \([-1, 1]\) and \(q > 0\), the maximum value of \(f(x)\) occurs when \(\cos(2x) = -1\):
\(f_{\max} = p - q(-1) = p + q = 9\).

The minimum value of \(f(x)\) occurs when \(\cos(2x) = 1\):
\(f_{\min} = p - q(1) = p - q = 3\).

Solving these simultaneous equations:
Adding the equations gives \(2p = 12 \implies p = 6\).
Subtracting the equations gives \(2q = 6 \implies q = 3\).

Thus, \(p = 6\) and \(q = 3\).

(b) We solve the inequality \(f(x) \ge 7.5\) for \(0 \le x \le \pi\):
\(6 - 3 \cos(2x) \ge 7.5\)
\(-3 \cos(2x) \ge 1.5\)
\(\cos(2x) \le -0.5\).

Let \(\theta = 2x\). Since \(0 \le x \le \pi\), we have \(0 \le \theta \le 2\pi\).

We solve \(\cos \theta \le -0.5\).

The boundary equation is \(\cos \theta = -0.5\).

The basic angle (reference angle) is \(\theta_{\text{basic}} = \cos^{-1}(0.5) = \frac{\pi}{3}\).

Since \(\cos \theta\) is negative, \(\theta\) lies in the second and third quadrants:
\(\theta_1 =
\pi - \frac{\pi}{3} = \frac{2\pi}{3}\)
\(\theta_2 = \pi + \frac{\pi}{3} = \frac{4\pi}{3}\).

The inequality \(\cos \theta \le -0.5\) is satisfied when:
\(\frac{2\pi}{3} \le \theta \le \frac{4\pi}{3}\).

Since \(\theta = 2x\):
\(\frac{2\pi}{3} \le 2x \le \frac{4\pi}{3} \implies \frac{\pi}{3} \le x \le \frac{2\pi}{3}\).

Marking scheme

(a)
M1: Realising that the maximum is \(p+q\) and the minimum is \(p-q\).
A1: Writing the simultaneous equations \(p+q = 9\) and \(p-q = 3\).
A1: Correctly solving to find \(p = 6\) and \(q = 3\).

(b)
M1: Setting up the inequality \(6 - 3\cos(2x) \ge 7.5\) and simplifying to \(\cos(2x) \le -0.5\).
M1: Solving \(\cos(2x) = -0.5\) to find at least one correct boundary angle in terms of \(\pi\) (e.g. \(2x = \frac{2\pi}{3}\) or \(2x = \frac{4\pi}{3}\)).
A1: Finding both correct boundary values for \(x\): \(x = \frac{\pi}{3}\) and \(x = \frac{2\pi}{3}\).
M1: For selecting the correct interval between the boundary values.
A1: Correct final inequality: \\frac{\pi}{3} \le x \le \frac{2\pi}{3}\\ (accept interval notation \([\frac{\pi}{3}, \frac{2\pi}{3}]\)).
Question 9 · long_structured
14.5 marks
A curve \( C \) has equation \( y = f(x) \), where \( f'(x) = \frac{16}{(2x-5)^3} - 2 \) for \( x > 2.5 \).

(a) Given that the curve passes through the point \( (3, 5) \), find the equation of the curve. [5]

(b) Find the coordinates of the stationary point of the curve and determine its nature. [4.5]

(c) A function \( g \) is defined by \( g(x) = 2x^2 - 14x + 31 \) for \( x \le k \).
Express \( g(x) \) in the form \( a(x-b)^2 + c \). [2]

(d) State the largest value of the constant \( k \) for which \( g \) has an inverse, and for this value of \( k \), find an expression for \( g^{-1}(x) \). [3]
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Worked solution

(a) To find the equation of the curve, we integrate \( f'(x) \):
\( f(x) = \int \left( 16(2x-5)^{-3} - 2 \right) dx \)
\( f(x) = \frac{16(2x-5)^{-2}}{-2 \times 2} - 2x + C \)
\( f(x) = -\frac{4}{(2x-5)^2} - 2x + C \)
Substitute the point \( (3, 5) \) to find the constant of integration \( C \):
\( 5 = -\frac{4}{(2(3)-5)^2} - 2(3) + C \)
\( 5 = -4 - 6 + C \implies C = 15 \)
So, the equation of the curve is:
\( f(x) = -\frac{4}{(2x-5)^2} - 2x + 15 \)

(b) To find the stationary point, set \( f'(x) = 0 \):
\( \frac{16}{(2x-5)^3} - 2 = 0 \implies (2x-5)^3 = 8 \implies 2x - 5 = 2 \implies x = 3.5 \)
Substitute \( x = 3.5 \) into \( f(x) \):
\( y = -\frac{4}{(2(3.5)-5)^2} - 2(3.5) + 15 = -1 - 7 + 15 = 7 \)
So the stationary point is \( (3.5, 7) \).
To determine its nature, find the second derivative:
\( f''(x) = \frac{d}{dx}\left(16(2x-5)^{-3} - 2\right) = -96(2x-5)^{-4} \)
At \( x = 3.5 \):
\( f''(3.5) = -96(2)^{-4} = -6 \)
Since \( f''(3.5) < 0 \), the stationary point is a maximum.

(c) Completing the square on \( g(x) \):
\( g(x) = 2(x^2 - 7x) + 31 \)
\( g(x) = 2\left(x - 3.5\right)^2 - 2(3.5)^2 + 31 \)
\( g(x) = 2(x - 3.5)^2 - 24.5 + 31 \)
\( g(x) = 2(x-3.5)^2 + 6.5 \)

(d) The quadratic function has its vertex at \( (3.5, 6.5) \). For \( g(x) \) to have an inverse, the domain must be restricted to a region where the function is one-to-one. Since the domain is defined for \( x \le k \), the largest possible value of \( k \) is the \( x \)-coordinate of the vertex:
\( k = 3.5 \)
To find the inverse, set \( y = 2(x-3.5)^2 + 6.5 \):
\( y - 6.5 = 2(x-3.5)^2 \implies \frac{y-6.5}{2} = (x-3.5)^2 \)
Since \( x \le 3.5 \), we take the negative square root:
\( x - 3.5 = -\sqrt{\frac{y-6.5}{2}} \implies x = 3.5 - \sqrt{\frac{y-6.5}{2}} \)
Hence, the inverse function is:
\( g^{-1}(x) = 3.5 - \sqrt{\frac{x-6.5}{2}} \) (or equivalent, such as \( 3.5 - \sqrt{0.5x - 3.25} \)) for \( x \ge 6.5 \).

Marking scheme

(a)
- M1: Attempt to integrate \( f'(x) \) with at least one power increased by 1.
- A1: Correct integration of the fractional term: \( -4(2x-5)^{-2} \).
- A1: Correct integration of the constant term: \( -2x \).
- M1: Substitute \( (3, 5) \) into an expression with a constant of integration \( C \).
- A1: Correct value of \( C = 15 \) and correct final equation.

(b)
- M1: Set \( f'(x) = 0 \) and solve for \( x \).
- A1: Obtain \( x = 3.5 \) (or equivalent).
- A1: Obtain \( y = 7 \).
- M1: Evaluate the second derivative (or alternative method) at \( x = 3.5 \).
- A0.5: Show \( f''(3.5) = -6 < 0 \) and state that it is a maximum.

(c)
- B1: Correctly write in the form \( 2(x - 3.5)^2 + c \).
- B1: Correctly find \( c = 6.5 \).

(d)
- B1: State \( k = 3.5 \) (accept \( \frac{7}{2} \)).
- M1: Make \( x \) the subject of their completed-square equation.
- A1: Correctly identify the negative square root to get \( g^{-1}(x) = 3.5 - \sqrt{\frac{x-6.5}{2}} \).
Question 10 · long_structured
14.5 marks
The curve \( C \) has equation \( y = \frac{4}{(2x-3)^{1.5}} \) for \( x > 1.5 \).

(a) Find the equation of the tangent to the curve \( C \) at the point where \( x = 2 \). [4.5]

(b) This tangent crosses the \( x \)-axis at \( A \) and the \( y \)-axis at \( B \). Find the area of the triangle \( OAB \), where \( O \) is the origin. [3]

(c) Find the volume of the solid generated when the region bounded by the curve \( C \), the \( x \)-axis, and the lines \( x = 2 \) and \( x = 4 \) is rotated through \( 360^\circ \) about the \( x \)-axis, giving your answer as an exact multiple of \( \pi \). [7]
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Worked solution

(a) When \( x = 2 \):
\( y = \frac{4}{(2(2)-3)^{1.5}} = \frac{4}{1^{1.5}} = 4 \)
So the point of contact is \( (2, 4) \).
Now differentiate \( y = 4(2x-3)^{-1.5} \) with respect to \( x \):
\( \frac{dy}{dx} = 4 \times (-1.5)(2x-3)^{-2.5} \times 2 \)
\( \frac{dy}{dx} = -12(2x-3)^{-2.5} \)
Substitute \( x = 2 \) into the derivative to find the gradient of the tangent:
\( m = -12(2(2)-3)^{-2.5} = -12 \)
Equation of the tangent:
\( y - 4 = -12(x-2) \)
\( y - 4 = -12x + 24 \implies 12x + y = 28 \)

(b) Find the intercepts of the tangent \( 12x + y = 28 \):
For \( A \) on the \( x \)-axis, set \( y = 0 \):
\( 12x = 28 \implies x = \frac{7}{3} \implies A = \left(\frac{7}{3}, 0\right) \)
For \( B \) on the \( y \)-axis, set \( x = 0 \):
\( y = 28 \implies B = (0, 28) \)
The area of the right-angled triangle \( OAB \) is:
\( \text{Area} = \frac{1}{2} \times OA \times OB = \frac{1}{2} \times \frac{7}{3} \times 28 = \frac{98}{3} \) (or \( 32\frac{2}{3} \))

(c) The formula for the volume of revolution about the \( x \)-axis is:
\( V = \pi \int_{a}^{b} y^2 dx \)
Here, \( y^2 = \left( \frac{4}{(2x-3)^{1.5}} \right)^2 = \frac{16}{(2x-3)^3} = 16(2x-3)^{-3} \).
\( V = \pi \int_{2}^{4} 16(2x-3)^{-3} dx \)
Integrate \( 16(2x-3)^{-3} \):
\( \int 16(2x-3)^{-3} dx = \frac{16(2x-3)^{-2}}{-2 \times 2} = -4(2x-3)^{-2} \)
Now evaluate between the limits \( 2 \) and \( 4 \):
\( V = \pi \left[ -\frac{4}{(2x-3)^2} \right]_{2}^{4} \)
At the upper limit \( x = 4 \):
\( -\frac{4}{(8-3)^2} = -\frac{4}{25} \)
At the lower limit \( x = 2 \):
\( -\frac{4}{(4-3)^2} = -4 \)
So the volume is:
\( V = \pi \left( -\frac{4}{25} - (-4) \right) = \pi \left( 4 - \frac{4}{25} \right) = \pi \left( \frac{96}{25} \right) = \frac{96}{25}\pi \) (or \( 3.84\pi \))

Marking scheme

(a)
- B1: Find the correct \( y \)-coordinate at \( x = 2 \) as \( 4 \).
- M1: Attempt to differentiate \( y = 4(2x-3)^{-1.5} \) using the chain rule.
- A1: Obtain correct derivative \( \frac{dy}{dx} = -12(2x-3)^{-2.5} \).
- M1: Substitute \( x = 2 \) to find gradient and formulate the equation of the tangent.
- A0.5: Obtain \( 12x + y = 28 \) (or equivalent form).

(b)
- M1: Identify \( x \) and \( y \) intercepts of their tangent line.
- A1: Correct coordinates \( A\left(\frac{7}{3}, 0\right) \) and \( B(0, 28) \) (or correct from their tangent).
- A1: Calculate the area of the triangle to get \( \frac{98}{3} \) (or exact equivalent).

(c)
- M1: Set up the volume integral \( V = \pi \int y^2 dx \).
- A1: Obtain correct integrand \( 16(2x-3)^{-3} \).
- M1: Attempt to integrate the power of \( (2x-3) \).
- A1: Correctly integrated expression: \( -4(2x-3)^{-2} \).
- M1: Show correct substitution of limits \( 4 \) and \( 2 \) into their integrated expression.
- A1: Correct numerical evaluation of the terms: \( -\frac{4}{25} \) and \( -4 \).
- A1: Obtain the final exact answer \( \frac{96}{25}\pi \) (or \( 3.84\pi \)).

Pure Mathematics 3 (Paper 35)

Answer all questions. Give non-exact numerical answers correct to 3 significant figures.
11 Question · 80.5 marks
Question 1 · structured
4.5 marks
Express the rational function \(\frac{2x^2 - 7x + 13}{(x - 1)(x - 2)^2}\) in partial fractions.
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Worked solution

Let \(\frac{2x^2 - 7x + 13}{(x - 1)(x - 2)^2} = \frac{A}{x - 1} + \frac{B}{x - 2} + \frac{C}{(x - 2)^2}\). Multiplying both sides by the denominator gives \(2x^2 - 7x + 13 = A(x - 2)^2 + B(x - 1)(x - 2) + C(x - 1)\). Substituting \(x = 1\) gives \(2(1)^2 - 7(1) + 13 = A(1 - 2)^2\), which simplifies to \(8 = A\), so \(A = 8\). Substituting \(x = 2\) gives \(2(2)^2 - 7(2) + 13 = C(2 - 1)\), which simplifies to \(7 = C\), so \(C = 7\). To find \(B\), compare the coefficients of \(x^2\): \(2 = A + B\). Since \(A = 8\), we have \(2 = 8 + B\), which gives \(B = -6\). Therefore, the partial fractions are \(\frac{8}{x - 1} - \frac{6}{x - 2} + \frac{7}{(x - 2)^2}\).

Marking scheme

M1: State a correct form of partial fractions with constants \(A\), \(B\), and \(C\). A1: Obtain \(A = 8\). A1: Obtain \(C = 7\). M1: Use a valid method to find \(B\), such as equating coefficients or substituting a third value of \(x\). A0.5: Obtain \(B = -6\) and state the final correct expression.
Question 2 · structured
4.5 marks
The complex number \(w\) is given by \(w = -4 + 4\sqrt{3}\mathrm{i}\). Find the three roots of the equation \(z^3 = w\), giving your answers in the form \(r\mathrm{e}^{\mathrm{i}\theta}\), where \(r > 0\) and \(-\pi < \theta \le \pi\).
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Worked solution

First, express \(w\) in polar form. The modulus of \(w\) is \(|w| = \sqrt{(-4)^2 + (4\sqrt{3})^2} = \sqrt{16 + 48} = 8\). The argument of \(w\) is \(\theta = \pi - \arctan\left(\frac{4\sqrt{3}}{4}\right) = \pi - \frac{\pi}{3} = \frac{2\pi}{3}\). Thus, \(w = 8\mathrm{e}^{\mathrm{i}\frac{2\pi}{3}}\). The equation \(z^3 = 8\mathrm{e}^{\mathrm{i}\left(\frac{2\pi}{3} + 2k\pi\right)}\) for integers \(k\) gives roots of the form \(z = 8^{1/3}\mathrm{e}^{\mathrm{i}\left(\frac{2\pi/3 + 2k\pi}{3}\right)}\). The modulus of each root is \(r = 8^{1/3} = 2\). Taking \(k = 0\), the argument is \(\frac{2\pi}{9}\), giving the root \(2\mathrm{e}^{\mathrm{i}\frac{2\pi}{9}}\). Taking \(k = 1\), the argument is \(\frac{2\pi/3 + 2\pi}{3} = \frac{8\pi}{9}\), giving the root \(2\mathrm{e}^{\mathrm{i}\frac{8\pi}{9}}\). Taking \(k = -1\), the argument is \(\frac{2\pi/3 - 2\pi}{3} = -\frac{4\pi}{9}\), giving the root \(2\mathrm{e}^{-\mathrm{i}\frac{4\pi}{9}}\).

Marking scheme

M1: Find the modulus of \(w\) as 8 and find the argument of \(w\) as \(\frac{2\pi}{3}\). A1: State the polar form of \(w\) correctly. M1: Use De Moivre's theorem or fractional index to find the modulus \(r = 2\) and set up the arguments. A1: Find two correct roots. A0.5: Find the third correct root in the specified range.
Question 3 · structured
4.5 marks
The polynomial \(p(x) = 3x^3 + ax^2 + bx - 10\), where \(a\) and \(b\) are constants, is divisible by \(x - 2\). When \(p(x)\) is divided by \(x + 1\), the remainder is \(-12\). Find the values of \(a\) and \(b\), and hence solve the inequality \(p(x) > 0\) for real values of \(x\).
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Worked solution

Using the factor theorem, since \(p(x)\) is divisible by \(x - 2\), we have \(p(2) = 0\), which gives \(3(2)^3 + a(2)^2 + b(2) - 10 = 0\). Simplifying, we get \(24 + 4a + 2b - 10 = 0\), so \(2a + b = -7\). Using the remainder theorem, since the remainder when divided by \(x + 1\) is \(-12\), we have \(p(-1) = -12\), which gives \(3(-1)^3 + a(-1)^2 + b(-1) - 10 = -12\). Simplifying, we get \(-3 + a - b - 10 = -12\), so \(a - b = 1\). Adding the two equations \(2a + b = -7\) and \(a - b = 1\) yields \(3a = -6\), so \(a = -2\). Substituting \(a = -2\) into \(a - b = 1\) gives \(b = -3\). The polynomial is therefore \(p(x) = 3x^3 - 2x^2 - 3x - 10\). Since \(x - 2\) is a factor, we can perform polynomial division or algebraic matching to get \(p(x) = (x - 2)(3x^2 + 4x + 5)\). For the quadratic factor \(3x^2 + 4x + 5\), the discriminant is \(4^2 - 4(3)(5) = 16 - 60 = -44 < 0\). Since the discriminant is negative and the leading coefficient is positive, \(3x^2 + 4x + 5 > 0\) for all real \(x\). Thus, \(p(x) > 0\) if and only if \(x - 2 > 0\), which gives \(x > 2\).

Marking scheme

M1: Use the factor theorem to set up an equation in terms of \(a\) and \(b\). M1: Use the remainder theorem to set up a second equation in terms of \(a\) and \(b\). A1: Solve the simultaneous equations to find \(a = -2\) and \(b = -3\). M1: Factorise the polynomial to obtain the quadratic factor \(3x^2 + 4x + 5\). A0.5: Show that the quadratic factor is always positive and conclude that the solution is \(x > 2\).
Question 4 · structured
4.5 marks
The complex number \(z\) satisfies the inequality \(|z - 3 - 4\mathrm{i}| \le 2\). Find the greatest and least values of \(|z|\), and find the greatest value of \(\arg z\) for points in this region, giving your answer in radians correct to 3 significant figures.
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Worked solution

The inequality \(|z - (3 + 4\mathrm{i})| \le 2\) represents a closed disc of radius \(R = 2\) centered at the point \(C\) representing \(3 + 4\mathrm{i}\). The distance from the origin to the center \(C\) is \(|OC| = \sqrt{3^2 + 4^2} = 5\). (i) The greatest value of \(|z|\) is \(|OC| + R = 5 + 2 = 7\). The least value of \(|z|\) is \(|OC| - R = 5 - 2 = 3\). (ii) The greatest value of \(\arg z\) occurs at the upper tangent line from the origin to the circle. Let \(\theta\) be the argument of the center \(C\), so \(\theta = \arctan\left(\frac{4}{3}\right) \approx 0.9273\) radians. Let \(\alpha\) be the angle between the line \(OC\) and the tangent line. In the right-angled triangle formed by the origin, the tangent point, and the center \(C\), we have \(\sin\alpha = \frac{\text{radius}}{|OC|} = \frac{2}{5} = 0.4\), which gives \(\alpha = \arcsin(0.4) \approx 0.4115\) radians. The greatest value of \(\arg z\) is \(\theta + \alpha = 0.9273 + 0.4115 = 1.3388 \approx 1.34\) radians.

Marking scheme

M1: Calculate the distance from the origin to the center of the circle as 5. A1: State the greatest value of \(|z|\) is 7 and the least value is 3. M1: Express the maximum argument as \(\theta + \alpha\), where \(\theta = \arg(C)\) and \(\sin\alpha = 0.4\). A1: Calculate \(\theta = 0.927\) and \(\alpha = 0.412\) (or equivalent in degrees then converted). A0.5: Obtain the final answer of 1.34 radians.
Question 5 · structured
4.5 marks
Solve the inequality \(|2x - 5| < 3|x + 1|\).
Show answer & marking scheme

Worked solution

Since both sides of the inequality are non-negative, we can square both sides: \((2x - 5)^2 < 9(x + 1)^2\). Expanding both sides gives \(4x^2 - 20x + 25 < 9(x^2 + 2x + 1)\), which simplifies to \(4x^2 - 20x + 25 < 9x^2 + 18x + 9\). Rearranging all terms to one side yields \(0 < 5x^2 + 38x - 16\). Factorising the quadratic expression, we get \((5x - 2)(x + 8) > 0\). The critical values are \(x = 0.4\) and \(x = -8\). Since we require the expression to be greater than zero, the solution set is \(x < -8\) or \(x > 0.4\).

Marking scheme

M1: Square both sides and attempt to expand. A1: Obtain the correct expanded inequality \(4x^2 - 20x + 25 < 9x^2 + 18x + 9\). M1: Rearrange to form a three-term quadratic and find critical values by factorising or using the formula. A1: Identify the critical values as \(-8\) and \(0.4\). A0.5: State the correct final range \(x < -8\) or \(x > 0.4\).
Question 6 · structured
8.5 marks
Two lines \( l_1 \) and \( l_2 \) are defined by the vector equations:

\[ l_1: \mathbf{r} = \mathbf{i} + 2\mathbf{j} - \mathbf{k} + \lambda(\mathbf{i} - \mathbf{j} + 2\mathbf{k}) \]
\[ l_2: \mathbf{r} = 4\mathbf{i} + 4\mathbf{j} + 3\mathbf{k} + \mu(2\mathbf{i} + 3\mathbf{j} + a\mathbf{k}) \]

where \( a \) is a constant.

(a) Find the value of \( a \) for which the two lines intersect. [4.5]

(b) For this value of \( a \), find the acute angle between the directions of the two lines, giving your answer correct to 1 decimal place in degrees. [4]
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Worked solution

(a) We equate the components of the two lines to find the point of intersection:

- \( x \)-components: \( 1 + \lambda = 4 + 2\mu \implies \lambda - 2\mu = 3 \) (Equation 1)
- \( y \)-components: \( 2 - \lambda = 4 + 3\mu \implies \lambda + 3\mu = -2 \) (Equation 2)

Subtracting Equation 2 from Equation 1:
\[ (\lambda - 2\mu) - (\lambda + 3\mu) = 3 - (-2) \]
\[ -5\mu = 5 \implies \mu = -1 \]

Substituting \( \mu = -1 \) back into Equation 1:
\[ \lambda - 2(-1) = 3 \implies \lambda = 1 \]

Now, we equate the \( z \)-components:
\[ -1 + 2\lambda = 3 + a\mu \]

Substituting \( \lambda = 1 \) and \( \mu = -1 \):
\[ -1 + 2(1) = 3 + a(-1) \]
\[ 1 = 3 - a \implies a = 2 \]

(b) The direction vectors of the two lines are \( \mathbf{d}_1 = \mathbf{i} - \mathbf{j} + 2\mathbf{k} \) and \( \mathbf{d}_2 = 2\mathbf{i} + 3\mathbf{j} + 2\mathbf{k} \).

Let \( \theta \) be the angle between the two direction vectors:
\[ \cos\theta = \frac{\mathbf{d}_1 \cdot \mathbf{d}_2}{|\mathbf{d}_1| |\mathbf{d}_2|} \]

Calculate the scalar product:
\[ \mathbf{d}_1 \cdot \mathbf{d}_2 = (1)(2) + (-1)(3) + (2)(2) = 2 - 3 + 4 = 3 \]

Calculate the magnitudes:
\[ |\mathbf{d}_1| = \sqrt{1^2 + (-1)^2 + 2^2} = \sqrt{6} \]
\[ |\mathbf{d}_2| = \sqrt{2^2 + 3^2 + 2^2} = \sqrt{4 + 9 + 4} = \sqrt{17} \]

Thus:
\[ \cos\theta = \frac{3}{\sqrt{6}\sqrt{17}} = \frac{3}{\sqrt{102}} \approx 0.29704 \]
\[ \theta = \arccos(0.29704) \approx 72.7^{\circ} \]

Marking scheme

(a)
- M1: Equate components to form at least two equations in \( \lambda \) and \( \mu \).
- A1: Solve the simultaneous equations to find correct values \( \lambda = 1 \) and \( \mu = -1 \).
- M1: Equate the third components using their values of \( \lambda \) and \( \mu \).
- A1.5: Obtain \( a = 2 \).

(b)
- M1: Use correct formula for the scalar product of the two direction vectors.
- A1: Obtain correct scalar product \( 3 \).
- A1: Calculate correct magnitudes \( \sqrt{6} \) and \( \sqrt{17} \).
- A1: Obtain final angle \( 72.7^{\circ} \) (accept \( 72.7 \)).
Question 7 · structured
8.5 marks
(a) Express \( 3\cos 2\theta - 4\sin 2\theta \) in the form \( R\cos(2\theta + \alpha) \), where \( R > 0 \) and \( 0 < \alpha < \frac{\pi}{2} \). Give the value of \( \alpha \) correct to 4 decimal places. [3.5]

(b) Hence solve the equation \( 3\cos 2\theta - 4\sin 2\theta = 2 \) for \( 0 < \theta < \pi \), giving your answers correct to 3 significant figures. [5]
Show answer & marking scheme

Worked solution

(a) We expand \( R\cos(2\theta + \alpha) \) using the compound angle identity:
\[ R\cos(2\theta + \alpha) = R\cos 2\theta \cos\alpha - R\sin 2\theta \sin\alpha \]

Comparing this with \( 3\cos 2\theta - 4\sin 2\theta \), we get:
\[ R\cos\alpha = 3 \]
\[ R\sin\alpha = 4 \]

We find \( R \):
\[ R = \sqrt{3^2 + 4^2} = \sqrt{25} = 5 \]

We find \( \alpha \):
\[ \tan\alpha = \frac{4}{3} \implies \alpha = \arctan\left(\frac{4}{3}\right) \approx 0.927295 \approx 0.9273 \text{ radians} \]

So the expression is \( 5\cos(2\theta + 0.9273) \).

(b) The equation becomes:
\[ 5\cos(2\theta + 0.9273) = 2 \]
\[ \cos(2\theta + 0.9273) = 0.4 \]

Given \( 0 < \theta < \pi \), we have:
\[ 0 < 2\theta < 2\pi \implies 0.9273 < 2\theta + 0.9273 < 2\pi + 0.9273 \approx 7.2105 \]

The principal value is:
\[ 2\theta + 0.9273 = \arccos(0.4) \approx 1.1593 \]

Since \( 1.1593 \) lies in the interval \( (0.9273, 7.2105) \):
\[ 2\theta = 1.1593 - 0.9273 = 0.2320 \implies \theta \approx 0.116 \text{ radians} \]

The second value in the interval is:
\[ 2\theta + 0.9273 = 2\pi - 1.1593 \approx 6.2832 - 1.1593 = 5.1239 \]
\[ 2\theta = 5.1239 - 0.9273 = 4.1966 \implies \theta \approx 2.10 \text{ radians} \]

Marking scheme

(a)
- B1: State \( R = 5 \).
- M1: Use \( \tan\alpha = \frac{4}{3} \) or equivalent.
- A1.5: Obtain \( \alpha = 0.9273 \) (or 0.927).

(b)
- B1: Rewrite the equation as \( \cos(2\theta + 0.9273) = 0.4 \).
- M1: Find at least one value of \( 2\theta + 0.9273 \) in the interval \( (0.9273, 7.2105) \).
- A1: Obtain \( \theta = 0.116 \).
- M1: Find the second valid angle value for \( 2\theta + 0.9273 \).
- A1: Obtain \( \theta = 2.10 \) (accept 2.1).
Question 8 · structured
8.5 marks
The point \( P \) has position vector \( 2\mathbf{i} + 5\mathbf{k} \). The line \( l \) has vector equation:

\[ \mathbf{r} = (2 + t)\mathbf{i} + (-1 + 2t)\mathbf{j} + (1 + 3t)\mathbf{k} \]

(a) Find the position vector of the point \( F \), the foot of the perpendicular from \( P \) to the line \( l \). [5.5]

(b) Find the exact perpendicular distance from \( P \) to \( l \). [3]
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Worked solution

(a) Let the foot of the perpendicular be \( F \). Since \( F \) lies on the line \( l \), its position vector is given by:
\[ \mathbf{f} = (2 + t)\mathbf{i} + (-1 + 2t)\mathbf{j} + (1 + 3t)\mathbf{k} \]

We find the vector \( \vec{PF} = \mathbf{f} - \mathbf{p} \):
\[ \vec{PF} = \begin{pmatrix} 2 + t \\ -1 + 2t \\ 1 + 3t \end{pmatrix} - \begin{pmatrix} 2 \\ 0 \\ 5 \end{pmatrix} = \begin{pmatrix} t \\ 2t - 1 \\ 3t - 4 \end{pmatrix} \]

The direction vector of the line \( l \) is \( \mathbf{d} = \mathbf{i} + 2\mathbf{j} + 3\mathbf{k} \).

Since \( PF \) is perpendicular to the line \( l \), the scalar product of \( \vec{PF} \) and \( \mathbf{d} \) must be zero:
\[ \vec{PF} \cdot \mathbf{d} = 0 \]
\[ t(1) + (2t - 1)(2) + (3t - 4)(3) = 0 \]
\[ t + 4t - 2 + 9t - 12 = 0 \]
\[ 14t - 14 = 0 \implies t = 1 \]

Substituting \( t = 1 \) back into the equation of \( l \) to find the position vector of \( F \):
\[ \mathbf{f} = (2 + 1)\mathbf{i} + (-1 + 2(1))\mathbf{j} + (1 + 3(1))\mathbf{k} = 3\mathbf{i} + \mathbf{j} + 4\mathbf{k} \]

(b) The perpendicular distance from \( P \) to \( l \) is the magnitude of \( \vec{PF} \).

Using \( t = 1 \), we have:
\[ \vec{PF} = \begin{pmatrix} 1 \\ 2(1) - 1 \\ 3(1) - 4 \end{pmatrix} = \begin{pmatrix} 1 \\ 1 \\ -1
\end{pmatrix} \]

Calculate the magnitude:
\[ |\vec{PF}| = \sqrt{1^2 + 1^2 + (-1)^2} = \sqrt{3} \]

Marking scheme

(a)
- M1: Express a general vector from \( P \) to the line \( l \) in terms of \( t \).
- A1: Obtain correct vector \( \vec{PF} = \begin{pmatrix} t \\ 2t-1 \\ 3t-4 \end{pmatrix} \) or equivalent.
- M1: Set up the scalar product of \( \vec{PF} \) and the direction vector \( \begin{pmatrix} 1 \\ 2 \\ 3 \end{pmatrix} \) equal to zero.
- A1: Solve for \( t = 1 \).
- M1: Substitute their \( t \) into the equation of \( l \) to get coordinates/vector of \( F \).
- A0.5: Obtain \( 3\mathbf{i} + \mathbf{j} + 4\mathbf{k} \) (or equivalent column vector format).

(b)
- M1: Find vector \( \vec{PF} \) using their \( F \) and \( P \).
- M1: Apply distance formula to find magnitude of their vector \( \vec{PF} \).
- A1: Obtain exact value \( \sqrt{3} \).
Question 9 · structured
8.5 marks
(a) Prove the identity:

\[ \frac{\sin 3\theta}{\sin \theta} - \frac{\cos 3\theta}{\cos \theta} \equiv 2 \]

[4.5]

(b) Hence solve the equation:

\[ \frac{\sin 3\theta}{\sin \theta} - \frac{\cos 3\theta}{\cos \theta} = 4\cos^2 \theta - 1 \]

for \( 0^{\circ} < \theta < 180^{\circ} \). [4]
Show answer & marking scheme

Worked solution

(a) To prove the identity, we start with the Left Hand Side (LHS) and combine the terms over a common denominator:
\[ \text{LHS} = \frac{\sin 3\theta \cos \theta - \cos 3\theta \sin \theta}{\sin \theta \cos \theta} \]

We apply the compound angle formula \( \sin(A - B) = \sin A \cos B - \cos A \sin B \) to the numerator, with \( A = 3\theta \) and \( B = \theta \):
\[ \sin 3\theta \cos \theta - \cos 3\theta \sin \theta = \sin(3\theta - \theta) = \sin 2\theta \]

So the expression becomes:
\[ \text{LHS} = \frac{\sin 2\theta}{\sin \theta \cos \theta} \]

Now, we apply the double angle identity \( \sin 2\theta = 2\sin\theta\cos\theta \) to the numerator:
\[ \text{LHS} = \frac{2\sin\theta\cos\theta}{\sin\theta\cos\theta} \]

For \( \sin\theta \ne 0 \) and \( \cos\theta \ne 0 \), we can cancel the terms in the numerator and denominator:
\[ \text{LHS} = 2 \]

Hence, the identity is proved.

(b) Using the identity proved in part (a), we substitute \( 2 \) for the left-hand side of the equation:
\[ 2 = 4\cos^2\theta - 1 \]
\[ 4\cos^2\theta = 3 \]
\[ \cos^2\theta = 0.75 \]
\[ \cos\theta = \pm \frac{\sqrt{3}}{2} \]

We solve for \( \theta \) in the interval \( 0^{\circ} < \theta < 180^{\circ} \):
- For \( \cos\theta = \frac{\sqrt{3}}{2} \):
\[ \theta = 30^{\circ} \]
- For \( \cos\theta = -\frac{\sqrt{3}}{2} \):
\[ \theta = 180^{\circ} - 30^{\circ} = 150^{\circ} \]

Both solutions are valid as they are within the specified interval.

Marking scheme

(a)
- M1: Express LHS as a single fraction with denominator \( \sin\theta\cos\theta \).
- M1: Apply compound angle identity \( \sin(3\theta - \theta) \) correctly to the numerator.
- A1: Obtain \( \frac{\sin 2\theta}{\sin\theta\cos\theta} \).
- M1: Apply double angle identity \( \sin 2\theta = 2\sin\theta\cos\theta \) correctly.
- A0.5: Simplify to obtain \( 2 \) and state conclusion.

(b)
- B1: Use the identity from (a) to write the equation as \( 2 = 4\cos^2\theta - 1 \).
- M1: Solve for \( \cos\theta \) to obtain \( \cos\theta = \pm\frac{\sqrt{3}}{2} \) (or equivalent decimal).
- A1: Obtain \( \theta = 30^{\circ} \).
- A1: Obtain \( \theta = 150^{\circ} \).
Question 10 · long_differential_equation
12 marks
The variables \(x\) and \(y\) satisfy the differential equation
$$\frac{\text{d}y}{\text{d}x} = \frac{2y^2 \ln x}{x(4+y)}$$
for \(x > 0\) and \(y > 0\). It is given that \(y = 4\) when \(x = 1\).

(i) Solve the differential equation, showing that the solution can be written in the form \(\ln(y/4) + 1 - 4/y = (\ln x)^2\). [7]

(ii) Show that when \(x = e\), the equation in part (i) can be written as \(y = 4e^{4/y}\). [2]

(iii) Use the iterative formula \(y_{n+1} = 4e^{4/y_n}\) to find the value of \(y\) when \(x = e\) correct to 3 significant figures. Start with \(y_1 = 7.0\) and show the result of each iteration to 5 significant figures. [3]
Show answer & marking scheme

Worked solution

(i) We separate the variables in the differential equation:
$$\int \frac{4+y}{y^2} \text{d}y = \int \frac{2 \ln x}{x} \text{d}x$$

Integrating the left-hand side:
$$\int \left( \frac{4}{y^2} + \frac{1}{y} \right) \text{d}y = -\frac{4}{y} + \ln y$$

Integrating the right-hand side, we use the substitution \(u = \ln x\), so \(\text{d}u = \frac{1}{x} \text{d}x\):
$$\int 2u \text{d}u = u^2 + C = (\ln x)^2 + C$$

Equating the two sides gives:
$$-\frac{4}{y} + \ln y = (\ln x)^2 + C$$

Using the initial conditions \(x = 1\) and \(y = 4\):
$$-\frac{4}{4} + \ln 4 = (\ln 1)^2 + C \implies -1 + \ln 4 = C$$

Substituting \(C\) back into the equation:
$$-\frac{4}{y} + \ln y = (\ln x)^2 - 1 + \ln 4$$

Rearranging to group the logarithmic terms:
$$\ln y - \ln 4 + 1 - \frac{4}{y} = (\ln x)^2$$
$$\ln(y/4) + 1 - \frac{4}{y} = (\ln x)^2$$

(ii) When \(x = e\), we substitute \(x = e\) and use the fact that \(\ln e = 1\):
$$\ln(y/4) + 1 - \frac{4}{y} = (\ln e)^2 = 1$$
$$\ln(y/4) - \frac{4}{y} = 0$$
$$\ln(y/4) = \frac{4}{y}$$

Taking the exponential of both sides:
$$\frac{y}{4} = e^{4/y} \implies y = 4e^{4/y}$$

(iii) Using the iterative formula \(y_{n+1} = 4e^{4/y_n}\) with \(y_1 = 7.0\):

\(y_1 = 7.0\)
\(y_2 = 4 e^{4/7.0} = 7.0832\)
\(y_3 = 4 e^{4/7.08317} = 7.0358\)
\(y_4 = 4 e^{4/7.03584} = 7.0627\)
\(y_5 = 4 e^{4/7.06265} = 7.0474\)
\(y_6 = 4 e^{4/7.04743} = 7.0561\)
\(y_7 = 4 e^{4/7.05606} = 7.0512\)
\(y_8 = 4 e^{4/7.05116} = 7.0539\)
\(y_9 = 4 e^{4/7.05394} = 7.0524\)

Thus, the value of \(y\) is \(7.05\) correct to 3 significant figures.

Marking scheme

(i)
M1: Attempt to separate variables and integrate both sides.
A1: Obtain correct LHS term \(-\frac{4}{y} + \ln y\).
M1: Attempt integration of RHS using substitution or inspection.
A1: Obtain correct RHS term \((\ln x)^2\).
M1: Substitute \(x = 1, y = 4\) to find the constant \(C\).
A1: Obtain \(C = \ln 4 - 1\) (or equivalent).
A1: Obtain the given relation \(\ln(y/4) + 1 - 4/y = (\ln x)^2\) with clear intermediate steps.

(ii)
M1: Substitute \(x = e\) and apply \(\ln e = 1\).
A1: Show steps clearly to arrive at \(y = 4e^{4/y}\).

(iii)
M1: Calculate at least two iterations to 5 s.f. correctly.
A1: State the sequence of values \(7.0832\), \(7.0358\), \(7.0627\), etc.
A1: State final answer \(7.05\) correct to 3 s.f.
Question 11 · long_calculus_integration
12 marks
The curve \(C\) has equation \(y = \frac{\ln x}{x^3}\) for \(x \ge 1\). The region \(R\) is bounded by the curve \(C\), the \(x\)-axis, and the line \(x = e\).

(i) Find the exact coordinates of the stationary point on \(C\). [4]

(ii) Find the exact area of the region \(R\). [4]

(iii) Find the volume of the solid formed when the region \(R\) is rotated completely through \(2\pi\) radians about the \(x\)-axis. Give your answer in the form \(\pi(a - b e^{-5})\), where \(a\) and \(b\) are positive rational numbers to be found. [4]
Show answer & marking scheme

Worked solution

(i) To find the stationary point, we differentiate \(y = \frac{\ln x}{x^3}\) using the quotient rule:
$$\frac{\text{d}y}{\text{d}x} = \frac{\frac{1}{x} \cdot x^3 - \ln x \cdot 3x^2}{(x^3)^2} = \frac{x^2 - 3x^2 \ln x}{x^6} = \frac{1 - 3\ln x}{x^4}$$

Setting the derivative to 0 for a stationary point:
$$1 - 3\ln x = 0 \implies \ln x = \frac{1}{3} \implies x = e^{1/3}$$

Substituting \(x = e^{1/3}\) back into the equation of \(C\):
$$y = \frac{\ln(e^{1/3})}{(e^{1/3})^3} = \frac{1/3}{e} = \frac{1}{3e}$$

So the exact coordinates of the stationary point are \(\left(e^{1/3}, \frac{1}{3e}\right)\).

(ii) The area of the region \(R\) is given by:
$$\text{Area} = \int_{1}^{e} \frac{\ln x}{x^3} \text{d}x$$

We use integration by parts with \(u = \ln x\) and \(\text{d}v = x^{-3} \text{d}x\), which gives \(\text{d}u = \frac{1}{x} \text{d}x\) and \(v = -\frac{1}{2x^2}\):
$$\int \frac{\ln x}{x^3} \text{d}x = -\frac{\ln x}{2x^2} - \int -\frac{1}{2x^3} \text{d}x = -\frac{\ln x}{2x^2} - \frac{1}{4x^2}$$

Evaluating this from \(1\) to \(e\):
$$\left[ -\frac{\ln x}{2x^2} - \frac{1}{4x^2} \right]_{1}^{e} = \left( -\frac{\ln e}{2e^2} - \frac{1}{4e^2} \right) - \left( -\frac{\ln 1}{2(1)^2} - \frac{1}{4(1)^2} \right)$$
$$= \left( -\frac{1}{2e^2} - \frac{1}{4e^2} \right) - \left( 0 - \frac{1}{4} \right)$$
$$= -\frac{3}{4e^2} + \frac{1}{4} = \frac{1}{4} - \frac{3}{4e^2}$$

(iii) The volume \(V\) of the solid of revolution is given by:
$$V = \pi \int_{1}^{e} y^2 \text{d}x = \pi \int_{1}^{e} \frac{(\ln x)^2}{x^6} \text{d}x$$

We use integration by parts with \(u = (\ln x)^2\) and \(\text{d}v = x^{-6} \text{d}x\), which gives \(\text{d}u = \frac{2\ln x}{x} \text{d}x\) and \(v = -\frac{1}{5x^5}\):
$$\int \frac{(\ln x)^2}{x^6} \text{d}x = -\frac{(\ln x)^2}{5x^5} + \frac{2}{5} \int \frac{\ln x}{x^6} \text{d}x$$

Now we evaluate \(\int \frac{\ln x}{x^6} \text{d}x\) by parts again, with \(U = \ln x\) and \(\text{d}V = x^{-6} \text{d}x\), giving \(\text{d}U = \frac{1}{x}\text{d}x\) and \(V = -\frac{1}{5x^5}\):
$$\int \frac{\ln x}{x^6} \text{d}x = -\frac{\ln x}{5x^5} - \int -\frac{1}{5x^6} \text{d}x = -\frac{\ln x}{5x^5} - \frac{1}{25x^5}$$

Substituting this back into the main integral:
$$\int \frac{(\ln x)^2}{x^6} \text{d}x = -\frac{(\ln x)^2}{5x^5} - \frac{2\ln x}{25x^5} - \frac{2}{125x^5}$$

Evaluating this from \(1\) to \(e\):
$$\text{At } x = e: \quad -\frac{1}{5e^5} - \frac{2}{25e^5} - \frac{2}{125e^5} = \frac{-25 - 10 - 2}{125e^5} = -\frac{37}{125e^5}$$
$$\text{At } x = 1: \quad 0 - 0 - \frac{2}{125} = -\frac{2}{125}$$

Subtracting the lower limit from the upper limit:
$$\int_{1}^{e} \frac{(\ln x)^2}{x^6} \text{d}x = -\frac{37}{125} e^{-5} - \left(-\frac{2}{125}\right) = \frac{2}{125} - \frac{37}{125} e^{-5}$$

Multiplying by \(\pi\):
$$V = \pi \left( \frac{2}{125} - \frac{37}{125} e^{-5} \right)$$

Here, \(a = \frac{2}{125}\) and \(b = \frac{37}{125}\).

Marking scheme

(i)
M1: Attempt to differentiate \(y\) using quotient rule (or product rule).
A1: Obtain correct derivative \(\frac{1-3\ln x}{x^4}\).
M1: Set derivative to 0 and solve for \(x\).
A1: Obtain correct coordinates \(\left(e^{1/3}, \frac{1}{3e}\right)\) in exact form.

(ii)
M1: Express area as integral and attempt integration by parts with \(u = \ln x, \text{d}v = x^{-3}\text{d}x\).
A1: Obtain correct term \(-\frac{\ln x}{2x^2} - \frac{1}{4x^2}\).
M1: Apply limits \(1\) and \(e\) correctly.
A1: Obtain exact area \(\frac{1}{4} - \frac{3}{4e^2}\) (or equivalent).

(iii)
M1: Express volume as \(\pi \int y^2 \text{d}x\) and attempt first integration by parts.
A1: Obtain correct first-stage integration by parts expression.
M1: Perform the second integration by parts successfully and substitute limits.
A1: Obtain final answer \(\pi\left(\frac{2}{125} - \frac{37}{125}e^{-5}\right)\) (accept \(a = \frac{2}{125}, b = \frac{37}{125}\)).

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