We already know that x is equal to sine of y. So if we could rewrite this bottom expression in terms, instead of cosine of y, if we could use our trigonometric identities to rewrite it in terms of sine of y, then we'll be in good shape because x is equal to sine of y. Well, how can we do that? Well, we know from our trigonometric identities, we know that sine squared of y plus cosine squared of y is equal to 1. Or if we want to solve for cosine of y, subtract sine squared of y from both sides, we know that cosine squared of y is equal to 1 minus sine squared of y, or that cosine of y, just take the principal root of both sides, is equal to the principal root of 1 minus sine squared of y. So we can rewrite this as being equal to 1 over, 1 over, instead of cosine of y, we could rewrite it as 1 minus sine squared of y. Now why is this useful?