Two angles are called complementary angles of each other, if their sum is the right angle $\displaystyle\frac{\pi}{2}$ i.e. $90^\circ$

For example, the acute angles of a right triangle are complement angles of each other, since the angle sum in any triangle is $180^\circ$

The sine of an angle is equal to the cosine of the complement angle, and vice versa.

The tangent of an angle equals to the cotangent of the complement angle, and vice versa (provided that no one of the angles is a multiple of the straight angle).