## The Lesson

Supplementary angles are two angles which add up to a straight angle (180°).Two angles that add to 180° are are said to supplement each other.## A Real Example of Supplementary Angles

Angles of 100° and 80° are supplementary angles.
100° + 80° = 180°

## How to Find a Supplementary Angle

## Question

What is the missing angle θ below?## Step-by-Step:

# 1

The 56° angle and θ are supplementary angles. They add up to 180°.

56° + θ = 180°

# 2

Subtract the 56° angle from 180°.

θ = 180° − 56° = 124°

## Answer:

The missing angle θ is 124°.## Top Tip

## What Types of Angles Are Supplementary?

**S**upplementary angles form a

**S**traight line.

## Note

## What's In an Name?

Supplementary comes from the Latin word "supplementum" meaning "fill up, complete, supply". This is because an angle is filled up to form a straight line. We get the word "supply" from the same root.## Supplementary Angles Can Be Acute, Right and Obtuse Angles

Supplementary angles can either be:- both right angles
- an acute angle and an obtuse angle.