To simplify the given expression, we can use the trigonometric identity:

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

Now, let's rewrite the expression:

sin^2(a)/cos^2(a) - sin^2(a)

= (sin^2(a) - sin^2(a)cos^2(a))/cos^2(a)

= sin^2(a)(1-cos^2(a))/cos^2(a)

= sin^2(a)sin^2(a)/cos^2(a)

= sin^4(a)/cos^2(a)