Edexcel IAL · Thinka 原創模擬試題

2025 Edexcel IAL Further Mathematics (YFM01) 模擬試題連答案詳解

Thinka Jan 2025 Cambridge International A Level-Style Mock — Further Mathematics (YFM01)

75 90 分鐘2025
An original Thinka practice paper modelled on the structure and difficulty of the Jan 2025 Cambridge International A Level Further Mathematics (YFM01) paper. Not affiliated with or reproduced from Cambridge.

部分 WFM01/01 Structure

Answer all nine questions. Show all working. Give non-exact answers to 3 significant figures.
9 題目 · 75
題目 1 · Structured Analytical
7.4
The complex numbers \(z_1\) and \(z_2\) are given by
\[z_1 = p + 4\mathrm{i}\]
\[z_2 = 3 - q\mathrm{i}\]
where \(p\) and \(q\) are real constants.
Given that
\[\frac{z_1}{z_2} = \frac{1}{2} + \frac{5}{2}\mathrm{i}\]

(a) Show that \(q = 7\). (3 marks)

(b) Find the value of \(p\). (2 marks)

(c) Find the exact value of \(|z_1|\). (2.4 marks)
查看答案詳解

解題

(a) We can rewrite the given division as multiplication:
\[z_1 = z_2 \left(\frac{1}{2} + \frac{5}{2}\mathrm{i}\right)\]
Substituting the expressions for \(z_1\) and \(z_2\):
\[p + 4\mathrm{i} = (3 - q\mathrm{i})\left(\frac{1}{2} + \frac{5}{2}\mathrm{i}\right)\]
\[p + 4\mathrm{i} = \frac{3}{2} + \frac{15}{2}\mathrm{i} - \frac{q}{2}\mathrm{i} - \frac{5q}{2}\mathrm{i}^2\]
Since \(\mathrm{i}^2 = -1\):
\[p + 4\mathrm{i} = \left(\frac{3 + 5q}{2}\right) + \left(\frac{15 - q}{2}\right)\mathrm{i}\]
Equating the imaginary parts of both sides:
\[4 = \frac{15 - q}{2} \implies 8 = 15 - q \implies q = 7\]

(b) Equating the real parts of both sides:
\[p = \frac{3 + 5q}{2}\]
Substituting \(q = 7\):
\[p = \frac{3 + 5(7)}{2} = \frac{38}{2} = 19\]

(c) Since \(z_1 = 19 + 4\mathrm{i}\), its modulus is:
\[|z_1| = \sqrt{19^2 + 4^2} = \sqrt{361 + 16} = \sqrt{377}\]

評分準則

(a)
M1: Writes down the equation as \(z_1 = z_2(0.5 + 2.5\mathrm{i})\) or multiplies numerator and denominator of \(\frac{p+4\mathrm{i}}{3-q\mathrm{i}}\) by \(3+q\mathrm{i}\) to equate to \(\frac{1}{2} + \frac{5}{2}\mathrm{i}\).
A1: Correctly expands to find the imaginary part of the product, yielding \(4 = \frac{15-q}{2}\) or equivalent.
A1*: Fully correct argument showing that \(q = 7\).

(b)
M1: Sets up a correct equation for the real part \(p = \frac{3+5q}{2}\) or equivalent, and substitutes their value of \(q\).
A1: Obtains \(p = 19\).

(c)
M1: Correctly applies the modulus formula \(|z_1| = \sqrt{a^2 + b^2}\) on their \(z_1\).
A1: Obtains the exact value \(\sqrt{377}\) (or equivalent exact surd).
題目 2 · Structured Analytical
7.4
The roots of the quadratic equation
\[2x^2 - 5x + 4 = 0\]
are \(\alpha\) and \(\beta\).
Without solving the equation:

(a) Find the value of \(\alpha^2 + \beta^2\). (2 marks)

(b) Find the value of \(\alpha^3 + \beta^3\). (2.4 marks)

(c) Form a quadratic equation, with integer coefficients, which has roots \(\frac{1}{\alpha^2}\) and \(\frac{1}{\beta^2}\). (3 marks)
查看答案詳解

解題

From the quadratic equation \(2x^2 - 5x + 4 = 0\), we have:
\[\alpha + \beta = \frac{5}{2}\]
\[\alpha\beta = 2\]

(a)
\[\alpha^2 + \beta^2 = (\alpha + \beta)^2 - 2\alpha\beta\]
\[\alpha^2 + \beta^2 = \left(\frac{5}{2}\right)^2 - 2(2) = \frac{25}{4} - 4 = \frac{9}{4}\]

(b)
Using the identity \(\alpha^3 + \beta^3 = (\alpha + \beta)^3 - 3\alpha\beta(\alpha + \beta)\):
\[\alpha^3 + \beta^3 = \left(\frac{5}{2}\right)^3 - 3(2)\left(\frac{5}{2}\right) = \frac{125}{8} - 15 = \frac{5}{8}\]

(c)
The new equation has roots \(\frac{1}{\alpha^2}\) and \(\frac{1}{\beta^2}\).
Sum of roots:
\[S = \frac{1}{\alpha^2} + \frac{1}{\beta^2} = \frac{\alpha^2 + \beta^2}{\alpha^2\beta^2} = \frac{\frac{9}{4}}{(\alpha\beta)^2} = \frac{\frac{9}{4}}{4} = \frac{9}{16}\]
Product of roots:
\[P = \frac{1}{\alpha^2} \times \frac{1}{\beta^2} = \frac{1}{(\alpha\beta)^2} = \frac{1}{4}\]
Using the equation template \(x^2 - Sx + P = 0\):
\[x^2 - \frac{9}{16}x + \frac{1}{4} = 0\]
Multiplying by 16 to get integer coefficients:
\[16x^2 - 9x + 4 = 0\]

評分準則

(a)
B1: States or uses both \(\alpha + \beta = \frac{5}{2}\) and \(\alpha\beta = 2\).
M1: Applies the identity \(\alpha^2 + \beta^2 = (\alpha + \beta)^2 - 2\alpha\beta\) with their values.
A1: Obtains \(\frac{9}{4}\) (or equivalent fraction/decimal).

(b)
M1: Uses a correct identity for \(\alpha^3 + \beta^3\) in terms of sum and product, or sum and sum of squares.
A1: Obtains \(\frac{5}{8}\) (or 0.625).

(c)
M1: Attempts to find the sum of the new roots, substituting their value from part (a) and \(\alpha\beta\).
M1: Attempts to find the product of the new roots, substituting \(\alpha\beta\).
A1: Formulates the correct equation \(16x^2 - 9x + 4 = 0\) (or any integer multiple thereof, including \(= 0\)).
題目 3 · Structured Analytical
7.4
The matrix \(\mathbf{M}\) is defined by
\[\mathbf{M} = \begin{pmatrix} a & 2a-3 \\ a+1 & a-2 \end{pmatrix}\]
where \(a\) is a real constant.
Given that \(\det(\mathbf{M}) = -9\):

(a) Find the two possible values of \(a\). (4 marks)

Using the positive value of \(a\) found in part (a):

(b) Find \(\mathbf{M}^{-1}\). (2 marks)

An original triangle \(T\) is transformed by \(\mathbf{M}\) onto an image triangle \(T'\).
Given that the area of \(T'\) is \(45\) square units,

(c) Find the area of the triangle \(T\). (1.4 marks)
查看答案詳解

解題

(a) We first find the determinant of \(\mathbf{M}\):
\[\det(\mathbf{M}) = a(a-2) - (2a-3)(a+1)\]
\[\det(\mathbf{M}) = (a^2 - 2a) - (2a^2 + 2a - 3a - 3)\]
\[\det(\mathbf{M}) = a^2 - 2a - (2a^2 - a - 3)\]
\[\det(\mathbf{M}) = -a^2 - a + 3\]
We are given that \(\det(\mathbf{M}) = -9\):
\[-a^2 - a + 3 = -9 \implies a^2 + a - 12 = 0\]
\[(a+4)(a-3) = 0\]
So, \(a = 3\) or \(a = -4\).

(b) Using the positive value, \(a = 3\), the matrix becomes:
\[\mathbf{M} = \begin{pmatrix} 3 & 3 \\ 4 & 1 \end{pmatrix}\]
Since \(\det(\mathbf{M}) = -9\), the inverse is given by:
\[\mathbf{M}^{-1} = \frac{1}{-9}\begin{pmatrix} 1 & -3 \\ -4 & 3 \end{pmatrix} = \begin{pmatrix} -\frac{1}{9} & \frac{1}{3} \\ \frac{4}{9} & -\frac{1}{3} \end{pmatrix}\]

(c) The area of the transformed triangle is related to the area of the original triangle by:
\[\text{Area}(T') = |\det(\mathbf{M})| \times \text{Area}(T)\]
\[45 = |-9| \times \text{Area}(T) = 9 \times \text{Area}(T)\]
\[\text{Area}(T) = \frac{45}{9} = 5\text{ square units}\]

評分準則

(a)
M1: Attempts to write an expression for \(\det(\mathbf{M})\) in terms of \(a\).
A1: Simplifies to find \(-a^2 - a + 3\).
M1: Sets their determinant expression equal to \(-9\) to form a quadratic equation and attempts to solve it.
A1: Obtains the correct values \(a = 3\) and \(a = -4\).

(b)
M1: Substitutes \(a=3\) into \(\mathbf{M}\) and attempts to find its inverse using a correct method.
A1: Obtains \(\mathbf{M}^{-1} = -\frac{1}{9}\begin{pmatrix} 1 & -3 \\ -4 & 3 \end{pmatrix}\) or equivalent.

(c)
M1: Recognises and applies the scale factor rule for area: \(\text{Area}(T') = |\det(\mathbf{M})| \times \text{Area}(T)\).
A1: Obtains the correct area of \(5\).
題目 4 · Structured Analytical
7.4
(a) Show that, for all positive integers \(n\),
\[\sum_{r=1}^{n} r(r+3) = \frac{1}{3}n(n+1)(n+5)\]
(4 marks)

(b) Hence, evaluate
\[\sum_{r=15}^{40} r(r+3)\]
(3.4 marks)
查看答案詳解

解題

(a) We expand the terms inside the summation:
\[\sum_{r=1}^{n} r(r+3) = \sum_{r=1}^{n} (r^2 + 3r) = \sum_{r=1}^{n} r^2 + 3\sum_{r=1}^{n} r\]
Using the standard summation formulae:
\[\sum_{r=1}^{n} r^2 = \frac{1}{6}n(n+1)(2n+1) \quad \text{and} \quad \sum_{r=1}^{n} r = \frac{1}{2}n(n+1)\]
Substituting these into the equation:
\[\sum_{r=1}^{n} r(r+3) = \frac{1}{6}n(n+1)(2n+1) + 3\left(\frac{1}{2}n(n+1)\right)\]
\[= \frac{1}{6}n(n+1)(2n+1) + \frac{9}{6}n(n+1)\]
Factorising out the common factor \[\frac{1}{6}n(n+1)\]:
\[= \frac{1}{6}n(n+1)\left[(2n+1) + 9\right]\]
\[= \frac{1}{6}n(n+1)(2n+10)\]
\[= \frac{2}{6}n(n+1)(n+5)\]
\[= \frac{1}{3}n(n+1)(n+5)\quad \text{(as required)}\]

(b) Using the result from part (a):
\[\sum_{r=15}^{40} r(r+3) = \sum_{r=1}^{40} r(r+3) - \sum_{r=1}^{14} r(r+3)\]
Let \(f(n) = \frac{1}{3}n(n+1)(n+5)\).
Evaluating for \(n = 40\):
\[f(40) = \frac{1}{3}(40)(41)(45) = 40 \times 41 \times 15 = 24600\]
Evaluating for \(n = 14\):
\[f(14) = \frac{1}{3}(14)(15)(19) = 14 \times 5 \times 19 = 1330\]
Thus:
\[\sum_{r=15}^{40} r(r+3) = 24600 - 1330 = 23270\]

評分準則

(a)
M1: Expands and splits the summation into \(\sum r^2 + 3\sum r\) and substitutes correct standard formulae.
M1: Factorises out a common factor of at least \(\frac{1}{6}n(n+1)\) or \(\frac{1}{2}n(n+1)\).
A1: Correctly simplifies the remaining factor to \((2n+10)\).
A1*: Reaches the printed answer \(\frac{1}{3}n(n+1)(n+5)\) with no errors in algebraic manipulation.

(b)
M1: Expresses the required sum as the difference between two sums from \(r=1\): \(\sum_{r=1}^{40} - \sum_{r=1}^{14}\).
M1: Substitutes \(n = 40\) and \(n = 14\) correctly into the formula derived in part (a).
A1: Calculates the final answer as \(23270\).
題目 5 · Structured Analytical
7.4
The function \(f\) is defined by
\[f(x) = x^3 - 5x + 3\]
The equation \(f(x) = 0\) has a root \(\alpha\) in the interval \([0, 1]\).

(a) Explain why there must be a root \(\alpha\) in the interval \([0, 1]\). (2 marks)

(b) Use linear interpolation once on the interval \([0, 1]\) to find an approximation to \(\alpha\). Give your answer to 3 decimal places. (2.4 marks)

(c) Taking \(0.5\) as a first approximation to \(\alpha\), apply the Newton-Raphson procedure once to \(f(x)\) to find a second approximation to \(\alpha\). Give your answer to 3 decimal places. (3 marks)
查看答案詳解

解題

(a) Evaluating the function at the boundaries of the interval:
\[f(0) = 0^3 - 5(0) + 3 = 3\]
\[f(1) = 1^3 - 5(1) + 3 = -1\]
Since \(f(x)\) is a polynomial, it is continuous on \([0, 1]\). Since there is a change of sign between \(f(0) = 3\) and \(f(1) = -1\), there must be at least one root \(\alpha\) in the interval \([0, 1]\).

(b) Using linear interpolation on \([0, 1]\):
\[\frac{x_1 - 0}{1 - x_1} = \frac{f(0)}{-f(1)}\]
\[\frac{x_1}{1 - x_1} = \frac{3}{-(-1)} = 3\]
\[x_1 = 3(1 - x_1) = 3 - 3x_1\]
\[4x_1 = 3 \implies x_1 = 0.75\]
To 3 decimal places, this is \(0.750\).

(c) First, we find the derivative of \(f(x)\):
\[f'(x) = 3x^2 - 5\]
Using \(x_0 = 0.5\):
\[f(0.5) = (0.5)^3 - 5(0.5) + 3 = 0.125 - 2.5 + 3 = 0.625\]
\[f'(0.5) = 3(0.5)^2 - 5 = 0.75 - 5 = -4.25\]
Applying the Newton-Raphson formula:
\[x_1 = x_0 - \frac{f(x_0)}{f'(x_0)}\]
\[x_1 = 0.5 - \frac{0.625}{-4.25} = 0.5 + \frac{5}{34} \approx 0.6470588...\]
To 3 decimal places, the approximation is \(0.647\).

評分準則

(a)
M1: Evaluates both boundary values \(f(0)\) and \(f(1)\) with at least one correct calculation.
A1: Concludes with a correct statement highlighting both the sign change and continuity of \(f(x)\).

(b)
M1: Attempts to set up a linear interpolation equation (e.g., using ratios or similar triangles).
A1: Solves to find the exact value \(x_1 = 0.75\).
A0.4: Correctly rounds to 3 decimal places as \(0.750\).

(c)
M1: Differentiates \(f(x)\) to get \(f'(x) = 3x^2 - 5\).
M1: Applies the Newton-Raphson formula \(x_1 = x_0 - \frac{f(x_0)}{f'(x_0)}\) using their values for \(f(0.5)\) and \(f'(0.5)\).
A1: Obtains \(0.647\) (rounded correctly to 3 d.p.).
題目 6 · Multi-part Proof/Derivation
9.5
The matrix \(\mathbf{M}\) is given by

\[\mathbf{M} = \begin{pmatrix} 3 & -4 \\ 1 & -1 \end{pmatrix}\]

(a) Prove by mathematical induction that for all \(n \in \mathbb{Z}^+\),

\[\mathbf{M}^n = \begin{pmatrix} 2n + 1 & -4n \\ n & 1 - 2n \end{pmatrix}\]

(b) Show that the matrix \(\mathbf{M}^n\) is non-singular for all \(n \in \mathbb{Z}^+\).
查看答案詳解

解題

(a) Let \(P(n)\) be the statement that \(\mathbf{M}^n = \begin{pmatrix} 2n + 1 & -4n \\ n & 1 - 2n \end{pmatrix}\).

**Base case:** For \(n = 1\),

\[\mathbf{M}^1 = \begin{pmatrix} 2(1) + 1 & -4(1) \\ 1 & 1 - 2(1) \end{pmatrix} = \begin{pmatrix} 3 & -4 \\ 1 & -1 \end{pmatrix}\]

Since this matches the given matrix \(\mathbf{M}\), \(P(1)\) is true.

**Inductive step:** Assume that \(P(k)\) is true for some positive integer \(k\). That is,

\[\mathbf{M}^k = \begin{pmatrix} 2k + 1 & -4k \\ k & 1 - 2k \end{pmatrix}\]

We must show that \(P(k+1)\) is true, i.e.,

\[\mathbf{M}^{k+1} = \begin{pmatrix} 2(k+1) + 1 & -4(k+1) \\ k+1 & 1 - 2(k+1) \end{pmatrix} = \begin{pmatrix} 2k + 3 & -4k - 4 \\ k + 1 & -2k - 1 \end{pmatrix}\]

Using matrix multiplication:

\[\mathbf{M}^{k+1} = \mathbf{M}^k \mathbf{M} = \begin{pmatrix} 2k + 1 & -4k \\ k & 1 - 2k \end{pmatrix} \begin{pmatrix} 3 & -4 \\ 1 & -1 \end{pmatrix}\]

Multiplying the matrices:

\[\mathbf{M}^{k+1} = \begin{pmatrix} (2k + 1)(3) + (-4k)(1) & (2k + 1)(-4) + (-4k)(-1) \\ k(3) + (1 - 2k)(1) & k(-4) + (1 - 2k)(-1) \end{pmatrix}\]

Simplifying each element:

- Row 1, Column 1: \(3(2k + 1) - 4k = 6k + 3 - 4k = 2k + 3 = 2(k+1) + 1\)
- Row 1, Column 2: \(-4(2k + 1) + 4k = -8k - 4 + 4k = -4k - 4 = -4(k+1)\)
- Row 2, Column 1: \(3k + 1 - 2k = k + 1\)
- Row 2, Column 2: \(-4k - (1 - 2k) = -4k - 1 + 2k = -2k - 1 = 1 - 2(k+1)\)

Thus,

\[\mathbf{M}^{k+1} = \begin{pmatrix} 2(k+1) + 1 & -4(k+1) \\ k+1 & 1 - 2(k+1) \end{pmatrix}\]

Since \(P(1)\) is true, and \(P(k) \implies P(k+1)\), the statement \(P(n)\) is true for all \(n \in \mathbb{Z}^+\) by mathematical induction.

(b) A matrix is non-singular if its determinant is non-zero.

\[\det(\mathbf{M}^n) = (2n + 1)(1 - 2n) - (-4n)(n)\]

\[\det(\mathbf{M}^n) = (1 - 4n^2) + 4n^2 = 1\]

Since \\det(\mathbf{M}^n) = 1
eq 0\) for all \(n \in \mathbb{Z}^+\), the matrix \(\mathbf{M}^n\) is non-singular for all \(n \in \mathbb{Z}^+\).

評分準則

(a)
B1: Verifies the formula for \(n = 1\).
M1: Assumes formula holds for \(n = k\) and attempts to multiply \(\mathbf{M}^k\) by \(\mathbf{M}\) (or vice versa).
A1: Correct multiplication expression shown.
A1: Correctly simplifies the top-left and bottom-left elements in terms of \(k+1\).
A1: Correctly simplifies the top-right and bottom-right elements in terms of \(k+1\).
A1: Complete and clear conclusion stating that since it is true for \(n=1\), and if true for \(n=k\) then it is true for \(n=k+1\), hence it is true for all \(n \in \mathbb{Z}^+\) by induction.

(b)
M1: Attempts to find the determinant of \(\mathbf{M}^n\) using the formula \(ad - bc\).
A1: Shows that \(\det(\mathbf{M}^n) = 1\) after simplification.
A1: Concludes with a valid reason that since \(\det(\mathbf{M}^n) = 1 \neq 0\), the matrix is non-singular for all \(n \in \mathbb{Z}^+\).
題目 7 · Multi-part Proof/Derivation
9.5
The point \(P(ap^2, 2ap)\), where \(p > 0\) and \(a > 0\), lies on the parabola \(C\) with equation \(y^2 = 4ax\).

(a) Show, using calculus, that the equation of the tangent to \(C\) at the point \(P\) is

\[py = x + ap^2\]

(b) The tangent to \(C\) at \(P\) intersects the directrix of \(C\) at the point \(Q\).

Find, in terms of \(a\) and \(p\), the coordinates of \(Q\).

(c) Show that the line \(SQ\), where \(S\) is the focus of \(C\), is perpendicular to the line \(SP\).
查看答案詳解

解題

(a) Differentiating the equation of the parabola with respect to \(x\):

\[2y \frac{\mathrm{d}y}{\mathrm{d}x} = 4a \implies \frac{\mathrm{d}y}{\mathrm{d}x} = \frac{2a}{y}\]

At the point \(P(ap^2, 2ap)\), the gradient of the tangent is:

\[m_T = \frac{2a}{2ap} = \frac{1}{p}\]

The equation of the tangent at \(P\) is:

\[y - 2ap = \frac{1}{p}(x - ap^2)\]

Multiplying by \(p\):

\[py - 2ap^2 = x - ap^2 \implies py = x + ap^2\]

(b) The equation of the directrix of \(C\) is \(x = -a\).

Substitute \(x = -a\) into the equation of the tangent:

\[py = -a + ap^2 \implies y = \frac{a(p^2 - 1)}{p}\]

Thus, the coordinates of \(Q\) are:

\[Q\left(-a, \frac{a(p^2 - 1)}{p}\right)\]

(c) The focus of the parabola \(C\) is \(S(a, 0)\).

The gradient of \(SP\) is:

\[m_{SP} = \frac{2ap - 0}{ap^2 - a} = \frac{2ap}{a(p^2 - 1)} = \frac{2p}{p^2 - 1}\]

The gradient of \(SQ\) is:

\[m_{SQ} = \frac{\frac{a(p^2 - 1)}{p} - 0}{-a - a} = \frac{a(p^2 - 1)}{p(-2a)} = \frac{p^2 - 1}{-2p} = -\frac{p^2 - 1}{2p}\]

Multiplying the two gradients:

\[m_{SP} \times m_{SQ} = \left(\frac{2p}{p^2 - 1}\right) \times \left(-\frac{p^2 - 1}{2p}\right) = -1\]

Since the product of their gradients is \(-1\), the line \(SQ\) is perpendicular to the line \(SP\).

評分準則

(a)
M1: Differentiates \(y^2 = 4ax\) implicitly to find an expression for \(\frac{\mathrm{d}y}{\mathrm{d}x}\).
A1: Finds the gradient of the tangent at \(P\) as \(\frac{1}{p}\).
M1: Uses the straight-line equation formula with gradient \(\frac{1}{p}\) and point \((ap^2, 2ap)\).
A1*: Fully simplifies to the given answer \(py = x + ap^2\) with no errors.

(b)
B1: Correctly identifies the equation of the directrix as \(x = -a\).
M1: Substitutes \(x = -a\) into their tangent equation and solves for \(y\).
A1: Correct coordinates of \(Q\left(-a, \frac{a(p^2 - 1)}{p}\right)\).

(c)
B1: Correctly identifies the focus \(S(a, 0)\).
M1: Formulates the gradient of \(SP\) and the gradient of \(SQ\) using \(\frac{y_2 - y_1}{x_2 - x_1}\).
A1: Obtains \(m_{SP} = \frac{2p}{p^2 - 1}\) and \(m_{SQ} = -\frac{p^2 - 1}{2p}\) in simplified forms.
A1: Shows that \(m_{SP} \times m_{SQ} = -1\) and concludes that the lines are perpendicular.
題目 8 · Multi-part Proof/Derivation
9.5
(a) Show that, for \(n \in \mathbb{Z}^+\),

\[\sum_{r=1}^{n} r(r+3) = \frac{1}{3}n(n+1)(n+5)\]

(b) Hence, show that

\[\sum_{r=n}^{2n} r(r+3) = \frac{7}{3}n(n+1)(n+2)\]
查看答案詳解

解題

(a) Expanding the term inside the summation:

\[\sum_{r=1}^{n} r(r+3) = \sum_{r=1}^{n} (r^2 + 3r) = \sum_{r=1}^{n} r^2 + 3\sum_{r=1}^{n} r\]

Using the standard summation formulas:

\[\sum_{r=1}^{n} r^2 = \frac{1}{6}n(n+1)(2n+1) \quad \text{and} \quad \sum_{r=1}^{n} r = \frac{1}{2}n(n+1)\]

Substitute these into the equation:

\[\sum_{r=1}^{n} r(r+3) = \frac{1}{6}n(n+1)(2n+1) + \frac{3}{2}n(n+1)\]

Factor out \(\frac{1}{6}n(n+1)\):

\[= \frac{1}{6}n(n+1) [ (2n+1) + 9 ]\]

\[= \frac{1}{6}n(n+1)(2n+10)\]

\[= \frac{2}{6}n(n+1)(n+5) = \frac{1}{3}n(n+1)(n+5)\]

This completes the proof for part (a).

(b) Using the result from part (a):

\[\sum_{r=n}^{2n} r(r+3) = \sum_{r=1}^{2n} r(r+3) - \sum_{r=1}^{n-1} r(r+3)\]

Using the formula with \(2n\) and \(n-1\) respectively:

\[\sum_{r=1}^{2n} r(r+3) = \frac{1}{3}(2n)(2n+1)(2n+5)\]

\[\sum_{r=1}^{n-1} r(r+3) = \frac{1}{3}(n-1)(n)(n+4)\]

Now, subtract the two expressions:

\[\sum_{r=n}^{2n} r(r+3) = \frac{1}{3}(2n)(2n+1)(2n+5) - \frac{1}{3}n(n-1)(n+4)\]

Factor out \(\frac{1}{3}n\):

\[= \frac{1}{3}n [ 2(2n+1)(2n+5) - (n-1)(n+4) ]\]

Expand the terms inside the square brackets:

\[2(2n+1)(2n+5) = 2(4n^2 + 12n + 5) = 8n^2 + 24n + 10\]

\[(n-1)(n+4) = n^2 + 3n - 4\]

So the expression becomes:

\[= \frac{1}{3}n [ (8n^2 + 24n + 10) - (n^2 + 3n - 4) ]\]

\[= \frac{1}{3}n [ 7n^2 + 21n + 14 ]\]

Factor out 7 from the quadratic term:

\[= \frac{7}{3}n(n^2 + 3n + 2)\]

Factorizing \(n^2 + 3n + 2 = (n+1)(n+2)\):

\[= \frac{7}{3}n(n+1)(n+2)\]

This completes the proof for part (b).

評分準則

(a)
M1: Expands the summand and splits the summation into two parts: \(\sum r^2 + 3\sum r\).
M1: Applies standard formulae for \(\sum r^2\) and \(\sum r\).
M1: Attempts to factorize out common factor \(\frac{1}{6}n(n+1)\) or \(\frac{1}{2}n(n+1)\).
A1*: Fully simplifies to get \(\frac{1}{3}n(n+1)(n+5)\) with no errors shown.

(b)
M1: Writes the sum \(\sum_{r=n}^{2n}\) as \(\sum_{r=1}^{2n} - \sum_{r=1}^{n-1}\).
A1: Writes correct substitution of \(2n\) and \(n-1\) into the formula from (a).
M1: Factors out \(\frac{1}{3}n\) and expands the remaining algebraic terms.
A1: Obtains the correct simplified quadratic expression \(7n^2 + 21n + 14\).
A1*: Correctly factorizes the expression to show \(\frac{7}{3}n(n+1)(n+2)\) with complete intermediate working.
題目 9 · Multi-part Proof/Derivation
9.5
The quadratic equation

\[2x^2 - 3x + 4 = 0\]

has roots \(\alpha\) and \(\beta\).

Without solving the equation:

(a) Show that

\[\alpha^3 + \beta^3 = -\frac{45}{8}\]

(b) Show that a quadratic equation with roots \(\frac{1}{\alpha^3}\) and \(\frac{1}{\beta^3}\) is

\[64x^2 + 45x + 8 = 0\]
查看答案詳解

解題

(a) For the quadratic equation \(2x^2 - 3x + 4 = 0\), we have:

\[\alpha + \beta = -\frac{-3}{2} = \frac{3}{2}\]

\[\alpha\beta = \frac{4}{2} = 2\]

We know the identity:

\[\alpha^3 + \beta^3 = (\alpha + \beta)^3 - 3\alpha\beta(\alpha + \beta)\]

Substituting the values of \(\alpha + \beta\) and \(\alpha\beta\):

\[\alpha^3 + \beta^3 = \left(\frac{3}{2}\right)^3 - 3(2)\left(\frac{3}{2}\right)\]

\[\alpha^3 + \beta^3 = \frac{27}{8} - 9\]

\[\alpha^3 + \beta^3 = \frac{27 - 72}{8} = -\frac{45}{8}\]

This completes the proof for part (a).

(b) Let the roots of the new quadratic equation be \(\gamma = \frac{1}{\alpha^3}\) and \(\delta = \frac{1}{\beta^3}\).

The sum of the new roots is:

\[\gamma + \delta = \frac{1}{\alpha^3} + \frac{1}{\beta^3} = \frac{\alpha^3 + \beta^3}{\alpha^3\beta^3} = \frac{\alpha^3 + \beta^3}{(\alpha\beta)^3}\]

Substituting the values from part (a) and \(\alpha\beta = 2\):

\[\gamma + \delta = \frac{-\frac{45}{8}}{2^3} = \frac{-\frac{45}{8}}{8} = -\frac{45}{64}\]

The product of the new roots is:

\[\gamma\delta = \frac{1}{\alpha^3} \times \frac{1}{\beta^3} = \frac{1}{(\alpha\beta)^3}\]

Substituting \(\alpha\beta = 2\):

\[\gamma\delta = \frac{1}{2^3} = \frac{1}{8}\]

The new quadratic equation is given by:

\[x^2 - (\gamma + \delta)x + \gamma\delta = 0\]

\[x^2 - \left(-\frac{45}{64}\right)x + \frac{1}{8} = 0\]

\[x^2 + \frac{45}{64}x + \frac{1}{8} = 0\]

Multiplying the entire equation by 64 to obtain integer coefficients:

\[64x^2 + 45x + 8 = 0\]

This completes the proof for part (b).

評分準則

(a)
B1: States correct values of \(\alpha + \beta = \frac{3}{2}\) and \(\alpha\beta = 2\).
M1: Recalls or derives the correct identity \(\alpha^3 + \beta^3 = (\alpha + \beta)^3 - 3\alpha\beta(\alpha + \beta)\).
M1: Substitutes their values of sum and product into the identity.
A1*: Obtains the exact value \(-\frac{45}{8}\) with no errors.

(b)
M1: Expresses the sum of the new roots \(\frac{1}{\alpha^3} + \frac{1}{\beta^3}\) with a common denominator of \(\alpha^3\beta^3\).
A1: Calculates the correct sum of the new roots as \(-\frac{45}{64}\).
M1: Calculates the correct product of the new roots as \(\frac{1}{8}\).
M1: Forms a quadratic equation of the form \(x^2 - S x + P = 0\) with their sum and product.
A1*: Multiplies by 64 to obtain the correct final equation \(64x^2 + 45x + 8 = 0\).

想知道自己有幾分把握?

Thinka 是 DSE 學生用的 AI 練習應用程式,有無限量練習題、即時自動批改和詳細解題步驟。逾 100,000 名學生用它確認自己真的識,而不只是「以為識」。

想練更多類似題型?在 Thinka 無限量操練,即時知道答案。

免費開始練習