Trigonometry - Trigonometric Identities

In trigonometry, trigonometric identities are equalities that involve trigonometric functions and are true for every value of the occurring variables for which both sides of the equality are defined. Geometrically, these are identities involving certain functions of one or more angles.
The basic trigonometric identities are equations involving the trigonometric functions (sine, cosine, tangent, secant, cosecant, and cotangent) that are always true regardless of the variables involved. We can use these equations to manipulate expressions involving trig and make them easier to work with.