


TrigonometryTrigonometric IdentitiesIf the hypotenuse of a right angled triangle is 1 unit, then by the definitions of the sine and the cosine the opposite side is sin θ and the adjacent side is cos θ. By the Pythagoras Theorem, This is usually written as Dividing the above identity by cos^{2}θ, we get Divide the identity by sin ^{2}θ, we get
The three formulas given in the boxes above are known as trigonometric identities as they provide relationships between trig functions. These identities are useful in solving a variety of problems. There are more trig identities that are not as easy to establish. For any angle θ:
For any angles A and B:
These identities can be used to simplify algebraic expressions to solve them or differentiate or integrate them. << Finding the Length of a Side in Right Angled Triangles  Trigonometry Index  Using the Value of One Trig Function to Find Another >> 