Trigonometric functions

Summary

The trigonometric functions are the six functions sine, cosine, tangent, cotangent, secant and cosecant of a real angle. Sine and cosine give the coordinates of a point on the unit circle. The other four are quotients built from them, such as the tangent $\tan = \sin / \cos$. They are periodic, they describe rotations and oscillations, and they are the building blocks of Fourier series. In analysis they are defined by power series, which makes the geometric facts into theorems.