Supplementary Angles – Pairs That Add to 180°

Two angles are supplementary if their measures add up to exactly 180° – a straight angle. Just like complementary angles, supplementary angles don't need to be next to each other; the only rule is that the two measures sum to 180°. To find the supplement of a known angle, subtract it from 180°.

“Supplementary” comes from the Latin supplere, meaning “to supply” or “to complete” – a supplementary angle supplies exactly what's missing to complete a straight 180° line. A simple way to keep “complementary” and “supplementary” straight: C comes before S alphabetically, and 90 comes before 180 numerically.

Finding a Supplement

Real-Life Application

• Adjustable lamps: the hinge angle and its supplement together describe how far a lamp arm can fold.
• Road signs: some warning signs use supplementary angle pairs to show opposite directions.
• Door hinges: a door's open angle and its supplement together span a straight line.

Key Takeaways

Practice: Supplementary Angles