The pythagorean formula tells us that
[tex]\sin ^2x+\cos ^2x=1[/tex]
Then,
[tex]\sin ^2x=1-\cos ^2x[/tex]
Replacing this in the numerator, we obtain
[tex]\frac{\sin ^2x}{1-\cos ^2x}=\frac{1-\cos ^2x}{1-\cos ^2x}=1[/tex]
Then, the expression is equal to 1.
