In mathematics, trigonometric identities are equalities that involve trigonometric functions and are true for every value of the occurring variables where both sides of the equality are defined. Geometrically, these are identities involving certain functions of one or more angles. If it is true for all values of the angles, an equation involving trigonometric ratios of an angle is called trigonometric identity. The trigonometric identities between trigonometric functions are equations that are true for the only right-angled triangle. Each of the six trig functions is equal to its co-function evaluated at the complementary angle periodicity of trig functions. Sine, cosine, secant, and cosecant have period 2pi while tangent and cotangent have period pi. Identities for negative angles. This chapter mainly deals with critical trigonometric identities.