Simplify sin^2(x)/(1 - sin^2(x))

We know from the pythagorean theorem:

sin^2(x) + cos^2(x) = 1

Subtract sin^2(x) from each side and we get:
cos^2(x) = 1 - sin^2(x)

We can rewrite our original expression as:
sin^2(x)/cos^2(x)

But this expression equals tan^2(x)