Functions

CIE A-Level Mathematics· Unit 1: Pure Mathematics 1, Topic 2· 15 min read

1. Functions, Domain and Range★★☆☆☆⏱ 4 min

📘 Definition

Function

f(x)

A mapping from a set of inputs (domain) to a set of outputs where each input maps to exactly one output.

Example:

$f (x) = 2 x + 1$ is a function; $x^{2} + y^{2} = 1$ is not a function for all $x$ .

The vertical line test checks if a graph represents a function: any vertical line will intersect the graph at most once if it is a function.

The domain of a function is the set of all valid input values $x$ . The range is the set of all possible output values $f (x)$ .

📐 Worked Example

Find the domain and range of $f (x) = \frac{1}{x - 3}$ for $x \in R$ .

1
First, find any $x$ that makes the function undefined. Denominators cannot equal zero:
2
$x - 3 = 0 \implies x = 3$
3
All other real values of $x$ are valid, so the domain is:
4
$\text{Domain: } x \in \mathbb{R}, x \neq 3$
5
To find the range, note that $\frac{1}{x - 3}$ can never equal zero (the numerator is a non-zero constant 1). All other non-zero real values are possible:
6
$\text{Range: } f(x) \in \mathbb{R}, f(x) \neq 0$

Exam tip:

Always check for two common domain restrictions: denominators cannot be zero, and expressions under square roots cannot be negative.

2. Composite Functions★★★☆☆⏱ 5 min

📘 Definition

Composite Function

f \circ g = f(g(x))

A combination of two functions where the output of the inner function $g$ becomes the input of the outer function $f$ . This means apply $g$ first, then $f$ .

Example:

If $f (x) = x^{2}$ and $g (x) = x + 1$ , $f (g (x)) = (x + 1)^{2}$ .

To find the domain of a composite function $f (g (x))$ , two conditions must always be satisfied: 1. $x$ is in the domain of $g$ , 2. $g (x)$ is in the domain of $f$ .

📐 Worked Example

Given $f (x) = \frac{1}{x}$ , $x \neq = 0$ , and $g (x) = x - 4$ , $x \in R$ , find the domain of $f (g (x))$ .

1
First, confirm that for $f (g (x))$ to exist, $g (x)$ must be a valid input to $f$ :
2
$f$ is only defined for non-zero inputs, so we require $g (x) \neq = 0$ :
3
$x - 4 = 0 \implies x = 4$
4
Next, check the domain of $g$ : $g$ is defined for all real $x$ , so the only restriction is $x \neq = 4$ :
5
$\text{Domain: } x \in \mathbb{R}, x \neq 4$

Exam tip:

Order matters for composite functions: $f (g (x))$ is not the same as $g (f (x))$ in most cases. Always apply the inner function first.

3. Inverse Functions★★★☆☆⏱ 5 min

📘 Definition

Inverse Function

f^{-1}(x)

A function that reverses the mapping of an original one-to-one (injective) function. If $f$ maps $x$ to $y$ , $f^{- 1}$ maps $y$ back to $x$ .

Example:

If $f (x) = 2 x + 3$ , $f^{- 1} (x) = \frac{x - 3}{2}$ .

A function is one-to-one if it maps each input to a unique output. Use the horizontal line test to check: any horizontal line intersects the graph at most once for a one-to-one function. A key property: $domain of f^{- 1} = range of f$ , $range of f^{- 1} = domain of f$ .

📐 Worked Example

Find the inverse of $f (x) = 2 x - 5$ , $x \in R$ .

1
Step 1: Let $y = f (x)$ :
2
$y = 2x - 5$
3
Step 2: Rearrange the equation to make $x$ the subject:
4
$y + 5 = 2x \implies x = \frac{y + 5}{2}$
5
Step 3: Swap $x$ and $y$ to get $f^{- 1} (x)$ :
6
$f^{-1}(x) = \frac{x + 5}{2}$
7
Verify: $f (f^{- 1} (x)) = 2 (\frac{x + 5}{2}) - 5 = x$ , which confirms the inverse is correct.

4. Inverses with Restricted Domains★★★★☆⏱ 6 min

Many common functions like quadratics are not one-to-one over their entire domain. To get an inverse, we first restrict the domain to make the function one-to-one. This is one of the most frequently tested concepts in CIE 9709.

📐 Worked Example

Given $f (x) = x^{2}$ , $x \geq 0$ , find $f^{- 1} (x)$ and state its domain and range.

1
Step 1: Confirm $f (x)$ is one-to-one on the restricted domain. For $x \geq 0$ , it passes the horizontal line test, so an inverse exists.
2
Step 2: Let $y = x^{2}$ , rearrange for $x$ :
3
$y = x^2 \implies x = \pm \sqrt{y}$
4
Step 3: The original domain is $x \geq 0$ , so we only take the positive root:
5
$x = \sqrt{y}$
6
Step 4: Swap $x$ and $y$ to get the inverse:
7
$f^{-1}(x) = \sqrt{x}$
8
Step 5: State domain and range, using the inverse property:
9
$\text{Domain of } f^{-1} = \text{Range of } f = x \geq 0 \\ \text{Range of } f^{-1} = \text{Domain of } f = f^{-1}(x) \geq 0$

Exam tip:

Always pick the correct root (positive or negative) based on the original restricted domain. This is the most commonly missed mark in inverse function questions.

5. Transformations of Graphs★★★☆☆⏱ 6 min

📘 Definition

Transformation of y = f(x)

y = f(x)+a,\quad f(x+a),\quad af(x),\quad f(ax)

A change to the equation y = f(x) that moves or stretches its graph in a predictable way, without redrawing it point by point.

Example:

If $f (x) = x^{2}$ , then $y = f (x) + 3$ is the same parabola raised 3 units, and $y = f (x - 2)$ is that parabola shifted 2 units to the right.

One idea unlocks every transformation in this topic:

A change on the outside of $f$ acts vertically and does exactly what you expect. A change on the inside of $f$ (applied to $x$ before $f$ acts) works horizontally and does the opposite of what you expect.

That is why $f (x) + a$ moves the graph up (outside, intuitive) but $f (x + a)$ moves it left (inside, reversed).

Translations (shifts).

$y = f (x) + a$ : shift up by $a$ (down if $a < 0$ ). Outside $f$ , so vertical.
$y = f (x + a)$ : shift left by $a$ (right if $a < 0$ ). Inside $f$ , so horizontal and reversed.

Stretches.

$y = a f (x)$ : vertical stretch, scale factor $a$ . Every $y$ -value is multiplied by $a$ ; points on the $x$ -axis stay fixed.
$y = f (a x)$ : horizontal stretch, scale factor $\frac{1}{a}$ . Every $x$ -value is divided by $a$ ; points on the $y$ -axis stay fixed.

Reflections are just stretches with a negative factor: $y = - f (x)$ reflects in the $x$ -axis, and $y = f (- x)$ reflects in the $y$ -axis.

📐 Worked Example

The function $f$ is defined by $f (x) = x^{2}$ . Describe fully the single transformation that maps $y = f (x)$ onto each of (a) $y = f (x) + 4$ , (b) $y = f (x - 3)$ , (c) $y = 2 f (x)$ , (d) $y = f (2 x)$ .

1
(a) The $+ 4$ is outside $f$ , so it acts vertically and intuitively:
2
$y = f(x) + 4 = x^2 + 4 \quad\Rightarrow\quad \text{translation 4 units up}$
3
(b) The $- 3$ is inside $f$ , so it acts horizontally and in reverse — subtracting moves the graph to the right:
4
$y = f(x-3) = (x-3)^2 \quad\Rightarrow\quad \text{translation 3 units right}$
5
(c) The $2$ multiplies the whole function (outside), so it is a vertical stretch:
6
$y = 2f(x) = 2x^2 \quad\Rightarrow\quad \text{vertical stretch, scale factor 2}$
7
(d) The $2$ multiplies $x$ inside $f$ , so it is a horizontal stretch with the reciprocal factor:
8
$y = f(2x) = (2x)^2 = 4x^2 \quad\Rightarrow\quad \text{horizontal stretch, scale factor } \frac{1}{2}$

📐 Worked Example

The curve $y = f (x)$ passes through the point $(4, 3)$ . It is transformed to $y = f (2 x) + 1$ . Describe the two transformations and find the image of $(4, 3)$ .

1
Read off each change. $f (2 x)$ is inside $f$ → horizontal stretch, scale factor $\frac{1}{2}$ . The $+ 1$ is outside → translation 1 unit up. The horizontal change acts on the $x$ -coordinate, the vertical change on the $y$ -coordinate.
2
Horizontal stretch, factor $\frac{1}{2}$ , multiplies the $x$ -coordinate by $\frac{1}{2}$ :
3
$x:\quad 4 \times \frac{1}{2} = 2$
4
The $+ 1$ raises the $y$ -coordinate by 1:
5
$y:\quad 3 + 1 = 4$
6
So the image of the point is:
7
$(4,\, 3) \;\longmapsto\; (2,\, 4)$
8
Sanity check: a factor of 2 inside $f$ gives a stretch of $\frac{1}{2}$ , pulling points towards the $y$ -axis ( $x = 4 \to x = 2$ ) — never a stretch of 2. That is 'inside = reversed' in action.

Exam tip:

Two traps examiners rely on: $f (x + a)$ moves the graph opposite to the sign of $a$ (so $+ a$ is a shift left), and $f (a x)$ stretches by $\frac{1}{a}$ , not by $a$ . When unsure, track one easy point through the change and see where it lands.

6. Common Pitfalls

Wrong move:

Claiming $x^{2} + y^{2} = 1$ is a function of $x$

Why:

A single $x$ value maps to two different $y$ values, so it fails the vertical line test

Correct move:

Recognize this is a relation, not a function, because each $x$ between $- 1$ and $1$ has two outputs

Wrong move:

Forgetting to check the domain of the inner function for composites

Why:

Only checking the restriction on the outer function misses existing restrictions on the inner function

Correct move:

Always check two conditions: $x$ is in the domain of the inner function, and the inner output is valid for the outer function

Wrong move:

Swapping order of composite functions, writing $g (f (x))$ for $f (g (x))$

Why:

Misremembering that the notation $f (g (x))$ means the inner function $g$ is applied first

Correct move:

Always remember: inner function first, outer function second for composite functions

Wrong move:

Keeping both roots when finding the inverse of a restricted quadratic

Why:

The original domain restricts the sign of $x$ , so only one root is valid for the inverse

Correct move:

Check the original domain to select the correct positive or negative root for the inverse

Wrong move:

Reading $y = f (x + 3)$ as a translation 3 units to the right

Why:

The $+ 3$ is inside $f$ , applied to $x$ before the function acts, so it works horizontally and in reverse — the graph moves 3 units to the left

Correct move:

Use 'inside = horizontal = reversed', or set the bracket to zero ( $x + 3 = 0 \Rightarrow x = - 3$ ) to see the graph shifts to $x = - 3$ , i.e. 3 units left

7. Quick Reference Cheatsheet

Concept	Key Rule
Function	Each input maps to exactly one output (passes vertical line test)
Domain	All valid $x$ ; exclude values that make denominators zero / square roots negative
Composite $f (g (x))$	Apply $g$ first, then $f$ ; domain: $x \in dom (g), g (x) \in dom (f)$
Inverse $f^{- 1} (x)$	Only exists for one-to-one functions; $dom (f^{- 1}) = ran (f)$ ; graph reflected in $y = x$
Finding inverse	Let $y = f (x)$ 2. Rearrange for $x$ 3. Swap $x$ and $y$

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2023 · 1
Find range of restricted quadratic
2022 · 1
Find inverse of composite function
2021 · 1
Domain of composite function

Going deeper

SyllabusCIE 9709 A-Level Mathematics 2025-2027

What's Next

Functions are a foundational topic for almost all other areas of pure mathematics, including calculus, coordinate geometry, and trigonometry. Understanding domain and range helps you correctly interpret limits, derivatives, and integrals, while inverse functions are essential for working with logarithmic and inverse trigonometric functions. This topic frequently combines with transformations of functions in CIE 9709 Paper 1, so building a solid understanding here will help you tackle more complex questions later in the course.