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°.

Lesson Slides

The slider below shows another real example of supplementary angles.

Top Tip

What Types of Angles Are Supplementary?

Supplementary angles form a Straight 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: