Section 12.2: Complementary and Supplementary Angles

Property

If the sum of the measures of two angles is 180°, then the angles are supplementary.

If the sum of the measures of two angles is 90°, then the angles are complementary.

Examples

• An angle measures 70°. Its supplement is 180° - 70° = 110° because supplementary angles must add up to 180°.
• An angle measures 35°. Its complement is 90° - 35° = 55° because complementary angles must add up to 90°.

Property

To classify angle pairs: Add the measures and compare to $90°$ and $180°$.

• Complementary: $\text{angle}_1 + \text{angle}_2 = 90°$
• Supplementary: $\text{angle}_1 + \text{angle}_2 = 180°$
• Neither: sum is not $90°$ or $180°$

Examples