All functions, including trig functions, can be described as being even, odd, or neither. Knowing whether a trig function is even or odd can help you simplify an expression. These even-odd identities are helpful when you have an expression where the variable inside the trig function is negative (such as –x). The even-odd identities are as follows:

sin(–x) = –sinx csc(–x) = –cscx
cos(–x) = cosx sec(–x) = secx
tan(–x) = –tanx cot(–x) = –cotx