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Supplementary Angles (Mathematics Lesson)

Supplementary Angles

Supplementary angles are two angles which add up to a straight angle.



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 angle supplements the 56° angle below?



Step 1: The 56° angle and θ are supplementary angles. They add up to 180°.
56° + θ = 180°.

Step 2: Subtract the 56° angle from 180°.
θ = 180° - 56° = 124°.

The supplementary angle is 124°.

Another Real Example of Supplementary Angles

The slider below shows a real example of supplementary angles in mathematics; the interior and exterior angles of a polygon:
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Top Tip
WHAT TYPES OF ANGLES ARE SUPPLEMENTARY?

Supplementary angles form a Straight Line.

Note
WHAT'S IN A NAME?

Supplementary comes from the Latin word 'supplementum' meaning "filled up".

This is because an angle is filled up to form a straight line.

SUPPLEMENTARY ANGLES CAN BE ACUTE, RIGHT AND OBTUSE

Supplementary angles can either be: