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? Well, sine of y is just x. So this is the same, if we just substitute back in, and let me just write it that way so it's a little bit clearer. I could write it as sine y squared.