The Trigonometric functions are functions related to right triangles, which have an angle of 90°.

They are: sine(sin), cosine(cos) and tangent(tan).

where θ is the reference angle.

The Opposite side of the triangle will always be opposite to the reference angle, the Hypotenuse will always be the largest side of the triangle and the Adjacent side of the triangle will always be beside the reference angle.

A good way to me remember all the formulas is with the phrase SOH CAH TOA, in Brazil we didn’t had something like this to remember the formulas:

S of sin;

C of Cosine;

T of Tangent;

O of Opposite;

H of Hypotenuse;

A of Adjacent.

source: sin-cos-tan.svg