Two angles are complementary when they add up to exactly 90°. Together they form one right angle.
If you know one of the angles, finding the other is simple subtraction: 90° minus the angle you know.
Remember
"C" in Complementary comes before "S" in Supplementary — just like 90 comes before 180. Complementary = 90°.
Real World
If you cut a right-angle corner of a piece of paper diagonally, the two angles you create are complementary — they add back to 90°.
Feeling ready?
Put this concept to work with a practice workout.