In maths What is an identity?

An identity is a set of two (or more) quantities that are identical in every single way. Trigonometry is famous for identities. Two examples include: cos (-x) = cos x and sin (-x) = -sin x The values on either side of the equal sign are in every way the same exact thing, and thus, these are identities. Solving trigonemtric algebra problems requires the often clever use of complex identities, and many seemingly unsolvable problems are solved in this way.