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. The derivative with respect to x of the inverse cosine of x is, I think I lost that color, I'll do it in magenta, is equal to negative one over the square root of one minus x squared. So this is a neat thing. This right over here is a neat thing to know.