Math basics › Trigonometry
Trig identities
The Pythagorean one
sin²θ + cos²θ = 1. Everything else is built on this, and it is just Pythagoras on the unit circle.
Quotient and reciprocal
tan = sin/cos. csc, sec and cot are 1/sin, 1/cos and 1/tan.
Double angle
sin 2θ = 2 sinθ cosθ, and cos 2θ = cos²θ − sin²θ.