Complementary Angles

In geometry, complementary angles are angles whose measures sum to 90°. If the two complementary angles are adjacent (i.e. have a common vertex and share just one side) their non-shared sides form a right angle.

In Euclidean geometry, the two acute angles in a right triangle are complementary, because the sum of internal angles of a triangle is 180 degrees, and the right angle itself accounts for ninety degrees.

The adjective complementary is from Latin complementum, associated with the verb complere, "to fill up". An acute angle is "filled up" by its complement to form a right angle.

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Complementary Angles - Trigonometric Ratios
... The sine of an angle equals the cosine of its complementary angle ... It is therefore true that, if angles A and B are complementary, and ... The tangent of an angle equals the cotangent of its complementary angle ...