Well, this is the same thing as trying to find the derivative with respect to x of, well, tangent of x is just sine of x. Sine of x over cosine of x. And since it can be expressed as the quotient of two functions, we can apply the quotient rule here to evaluate this or to figure out what this is going to be. 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?