To simplify the expression, we can first expand the numerator using the difference of squares formula:

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

Now, we can substitute this back into the original expression:

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

Next, we can use the Pythagorean identity sin^2(a) + cos^2(a) = 1 to simplify the denominator:

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

Therefore, the simplified form of the expression is -1.