Trigonometry: Negative Angle Identities - Expii

Based on the unit circle, the negative angle identities (also called "odd/even" identities) tell you how to find the trig functions at -x in terms of the trig functions at x. In other words, they relate trig values at opposite angles x and -x. For example, sin(-x) = -sin(x), cos(-x) = cos(x), and tan(-x) = -tan(x).