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. This is equal to one minus cosine. I could write it like this, cosine squared of y, but I'll write it like this, because it'll make it a little bit clearer. And what is cosine of y?