The quotient rule tells us that this is going to be the derivative of the top function, which we know is cosine of x, times the bottom function, which is cosine of x, so times cosine of x, minus the top function, which is sine of x, the top function, which is sine of x, sine of x, times the derivative of the bottom function, so the derivative of cosine of x is negative sine of x. So I could put the sine of x there, but where the negative can just cancel that out, and it's going to be over, over the bottom function squared. So cosine squared of x. Now what is this? Well, what we have here, this is just cosine squared of x. This is just sine squared of x. And we know from the Pythagorean identity, and this really just comes out of the unit circle definition, that cosine squared of x plus sine squared of x, well that's going to be equal to one for any x.