The science which deals with the measurements of triangles is called trigonometry. Trigonometrically ratios and its relations are used to prove trigonometric identities. Further, the trigonometrically ratios of complementary angles and the use of trigonometrically tables are the other topics covered in this chapter. 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. There are six trigonometric ratios, sine, cosine, tangent, cosecant, secant and cotangent. These six trigonometric ratios are abbreviated as sin, cos, tan, csc, sec, cot. These are referred to as ratios since they can be expressed in terms of the sides of a right-angled triangle for a specific angle.