We want it in terms of x. And we know that x is cosine of y. So let's see if we can rewrite this bottom expression in terms of cosine of y instead of sine of y. Well, we know, and we saw it in the last video, that from the Pythagorean identity, that cosine squared of y plus sine squared of y is equal to one. We know that sine of y is equal to the square root of one minus cosine squared of y. So this is equal to negative one. This is just a manipulation of the Pythagorean trig identity.