# Evaluating a Function(KS4, Year 10)

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A function is a relation between a set of inputs and a set of outputs, such that each input is related to exactly one output. Evaluating a function means putting an input into a function, and finding the output it relates to.

## Understanding Evaluating a Function

To evaluate a function means to see what output an input is mapped to. The image below shows a mapping diagram which relates a set of inputs to a set of outputs. If we wanted to evaluate the function when the input is 2, we would see which output it is mapped to. Imagine we have the function below:If we evaluate the function at x = 2, we need to find f(2).

## A Real Example of How to Evaluate a Function

Evaluating a function is easy.

## Question

Evaluate the function f(x) = 2x + 1 at x = 2.

## 1

Write the function.
f(x) = 2x + 1
Note: 2x = 2 × x.

## 2

Substitute x = 2 into the function.

f(2) = 2 × 2 + 1

f(2) = 4 + 1

f(2) = 5

If we evaluate f(x) = 2x + 1 at x = 2, we get 5. f(2) = 5.

## Lesson Slides

The slider below another example of how to evaluate functions.

## A Note on Notation

If a function f is evaluated at a number (e.g. 2), this is denoted f(2).

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