Hello everyone! Welcome to the summary of the "Functions" chapter.

When you hear the word "Function," many of you might start frowning, but believe me, it’s actually a concept that surrounds us more than you think! For the A-Level Applied Mathematics 2 exam, this chapter is a core pillar of the Number and Algebra section because it serves as the foundation for so many other topics.

If it feels difficult at first, don't worry! Let's imagine a function as a "machine" where we put "raw materials" (Input) in, and it processes them into "finished products" (Output) according to a set formula. Ready? Let's dive in!

1. What exactly is a function?

In mathematics, a function is a special type of relation. Its golden rule is: "Each input ( \(x\)) must be paired with exactly one output ( \(y\))."

Let's compare:
Think of a "water vending machine."
- If you press button 1 ( \(x\)) and get plain water ( \(y\)), that is a function.
- If you press button 2 ( \(x\)) and also get plain water ( \(y\)), that is still a function (multiple buttons can lead to the same output).
- But! If you press button 3 ( \(x\)) and sometimes get a Coke, and other times get yogurt (one \(x\) gives two different \(y\) values), then it is not a function!

Key point: \(x\) must be loyal and pair with only one \(y\), but \(y\) is free to be "popular" and have multiple \(x\) values pairing with it.

2. Domain and Range

These two terms refer to the groups of inputs and outputs.

  • Domain ( \(D\)): The set of all \(x\) values that can be put into the function to yield a valid \(y\) value.
  • Range ( \(R\)): The set of all \(y\) values that come out of the function machine.

Common pitfalls:
There are two main rules to watch out for when finding the domain:
1. The denominator cannot be zero: For example, in \(f(x) = \frac{1}{x-2}\), \(x\) cannot be \(2\) because it would make the denominator \(0\), which is undefined.
2. Even roots cannot have a negative inside: For example, in \(f(x) = \sqrt{x-5}\), \(x\) must always be greater than or equal to \(5\) (the value inside the root must be \(\ge 0\)).

3. Linear Function

The simplest function that appears frequently on exams is one that forms a "straight line".
Equation form: \(f(x) = ax + b\) (or \(y = mx + c\))

  • \(a\) (or \(m\)): The Slope. If it’s positive, the graph points upward; if it’s negative, it points downward.
  • \(b\) (or \(c\)): The y-intercept (the value of \(y\) when \(x = 0\)).

Did you know? Straight-line graphs are everywhere in real life, such as taxi fare calculations (base fare + distance cost) or simple interest calculations. All these are linear functions!

4. Quadratic Function

If \(x\) starts having an exponent of \(2\), the graph is no longer a straight line—it becomes a "parabola".
Equation form: \(f(x) = ax^2 + bx + c\) where \(a \neq 0\)

Easy tricks for visualizing the graph:
- If \(a\) is positive (+): The graph is "opening upward" (a smiley face) -> there is a minimum point.
- If \(a\) is negative (-): The graph is "opening downward" (a frowny face) -> there is a maximum point.

Vertex of the graph:
We can find the \(x\) value at the vertex using the formula \(x = -\frac{b}{2a}\), then substitute \(x\) back into the function to find \(y\).

Real-world example: When throwing a basketball into the air, its trajectory is always a downward-opening parabola. The maximum height of the ball is simply the "maximum point" of this function.

5. Piecewise Functions

In real life and on the Applied Mathematics 2 exam, rules often change depending on the input. A piecewise function is a function defined by multiple formulas, where each formula applies to a specific interval of \(x\).
Common real-world context: Parcel shipping fees based on package weight tiers, electricity billing rates, or parking fees.

Example: Suppose a delivery service charges fees \(f(x)\) in Baht according to weight \(x\) in kilograms:
- \(f(x) = 50\) when \(0 < x \le 2\)
- \(f(x) = 50 + 20(x - 2)\) when \(2 < x \le 5\)
- \(f(x) = 150\) when \(x > 5\)

How to evaluate:
To find \(f(3.5)\), check which condition \(x = 3.5\) satisfies. Because \(2 < 3.5 \le 5\), use the second formula: \(f(3.5) = 50 + 20(3.5 - 2) = 50 + 30 = 80\) Baht. Exam problems frequently ask you to evaluate values across different intervals and sum or compare them.

6. Summary of steps to solve function problems

If you encounter a function problem on the exam, don't panic. Try these steps:

  1. See what you're given: Does the problem provide an equation, coordinates, or interval conditions for different cases?
  2. Substitute what you know: If the problem says \(f(2) = 5\), it means "If you replace \(x\) with \(2\), the final result ( \(y\)) must be \(5\)." If it is a piecewise function, find the interval that contains the input first.
  3. Draw a graph or visualize it: If it's a first-degree equation, it's a line; if it's a quadratic, it's a parabola. This helps you understand the bigger picture.
  4. Watch for conditions: Denominators cannot be \(0\), and the inside of roots cannot be negative.
Crucial points to memorize!

- Function means \(1\) input \(x\) pairs with only \(1\) output \(y\).
- \(f(x)\) is just the name of the function, which equals \(y\).
- Downward/Upward parabola: Look at the term in front of \(x^2\) (positive = opening upward, negative = opening downward).
- Maximum/Minimum point: Occurs at \(x = -\frac{b}{2a}\).
- Piecewise function: Always verify which interval \(x\) belongs to before substituting into a rule.

Finally... Mathematics isn't just about memorizing formulas; it's about practice. The more you solve function problems, the more you'll start to recognize patterns and work faster.
"Do a little bit, but do it every day, and you'll definitely improve!" Good luck to all future university students!