finding missing angles Pairs of angles that add up to 180º are called supplementary angles Any two angles which add up to 180º are known as supplementary angles. They are easiest to spot when they're on a straight line, but they can also be far apart. These two angles must be supplementary. 81º bº 81º bº They would still be supplementary even if they weren't touching, but we couldn't be sure without knowing what angle b is.