A trigonometric expression which is valid for all the angles is called a trigonometric identity.
Trigonometric ratios have several properties and they can express in mathematical form too. They are too for all the angles. Therefore, they are used as formulas in solving the mathematical problems and derivations.
Trigonometric ratios form various identities in trigonometry mathematically. The following list of trigonometric identities are often used in mathematics as formulas.