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. We know that this thing right over here is x. So this is going to be equal to, and we deserve a little bit of a drum roll, 1 over the square root of 1 minus, instead of sine of y, we know that x is equal to sine of y. So 1 minus x squared.