The three interior angles of every triangle add to 180°. To find a missing angle, subtract the known angles from 180°.
Example: a right triangle has angles 90° and 35°. The third angle is 180° − 90° − 35° = 55°.
These relationships can form equations. If complementary angles are x and 2x, then x + 2x = 90, so 3x = 90 and the angles are 30° and 60°.
Memory aid: complementary angles form a Corner (90°); supplementary angles form a Straight line (180°).
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