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? Well, of course, that is x. So this is equal to negative one over the square root of one minus, instead of writing cosine y, instead of writing cosine y, I'm trying to switch colors, instead of writing cosine y, we could write one minus x, one minus x squared. So there you have it.