Question

How do you prove `sin^2x + tan^2x/(1-tan^2x) = (sin^2x+cos^2x)/(cos^2x - sin^2x) -cos^2x`?

Answer 1

Are you sure that there isn't a mistake? As you can see here, it's not an identity: the two functions at the left and right member are two different functions, with a couple of intersections.

To have an identity, the two function should assume the same value for every `x`.