Verifying trig identities

Essentially to verify a trig identity you must use different forms of substituting trig functions. One major one is tan=sin/cos. Another common one is sin^2+cos^2=1. 1+tan^2=sec^2 and 1+cot^2=csc^2 are also very commonly used. In many ways, verifying trig identities is like solving a puzzle. You must chose the most complicated side to work one, and use only that side. Most of the times you start by putting everything in terms of sin and cos.