Trigonometry: Co-Function Identities - Expii

The co-function identities relate "co-pairs" of trig functions at x and π/2 - x. For example, sin(π/2 - x) = cos(x), cos(π/2 - x) = sin(x), tan(π/2 - x) = cot(x), and cot(π/2 - x) = tan(x).