We have already learnt that trigonometry is the study of relationships between the sides and angles of a triangle. The ratios of the sides in a right triangle with respect to its acute angles are called trigonometric ratios of the angle. The angles  and  are useful angles in trigonometry, and their numerical values are easy to remember.
When two angles add up to 90⁰, then any one angle is the complement of the other. Trigonometric ratios of complementary angles help in simplifying problems.