**Cos 90 X Identity**. The cofunction identities show the relationship between sine, cosine, tangent, cotangent, secant and cosecant. I know what you did last summer…Trigonometric Proofs.

In calculus and all its applications, the trigonometric identities are of central importance. A Trigonometric identity is an identity that contains the trigonometric functions sin, cos, tan, cot, sec or csc. Sine of angle is equal to the ratio of opposite side and hypotenuse whereas cosine of an angle is equal to ratio of adjacent side and hypotenuse.

### To this point we know the exact values of the trigonometric functions at only a few angles.

Additional identities can be derived from the sum and difference identities for cosine and sine.

Meanwhile trigonometric identities are equations that involve trigonometric functions that are always true. Trigonometric functions, identities, formulas and the sine and cosine laws are presented. We use an identity to give an expression a more convenient form.