Grade 8 · Unit 3Topic 1 of 2~22 min

Introduction to Functions

A function gives exactly one output for each input.

Step 1 · Learn

Walkthrough

Read this first — it covers the idea in plain language with a worked example.

One input, one output

A function is like a machine: you put in an input and get exactly one output. If a single input could give two different outputs, it is not a function.

The vertical line test

On a graph, if you can draw a vertical line that crosses the graph more than once, the relationship is not a function.

Worked example

Is the set {(1, 2), (3, 4), (1, 5)} a function?

Look at the inputs: 1 appears twice, with outputs 2 and 5.
Since one input has two different outputs, it is not a function.

Step 2 · Watch

Video walkthrough

A short video that walks through this topic.

Khan Academy

What Is a Function?

Inputs, outputs, and what makes a relationship a function.

Watch on YouTube

Step 3 · Practice

Practice

Try a worksheet or work through some problems before the quiz.

Khan Academy

Intro to functions

Open resource

Step 4 · Check

Quick check

A couple of questions to make sure this topic stuck. Retake as many times as you want.

Introduction to Functions

Question 1 of 2

Medium

Which set of ordered pairs is a function?

Back toUnit 3 overview Next up · Topic 2 of 2Linear & Nonlinear Functions