It's not an identity. Does tan^2x = 1-(cos2x/1)? I'm certain it doesn't. It does, however, equal to (1-cos2x)/2. This is an example of a half-angle identity.
the first i did was to use the double angle formula for sine and cosine. then it was simplified, then since the ratio of sine and cosine is tangent, then the next step is obvious.