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?StepbyStep:
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?
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:
both right angles

an acute angle and an obtuse angle.