So if we have g of h of x is equal to x, h of x is equal to x, so we put that h of x back there, which is by definition true if g and h's are inverses. Well now let's take the derivative of both sides of this. So let's take the derivative with respect to x of both sides, derivative with respect to x. And on the left hand side, well we just apply the chain rule. So this would be g prime of h of x, g prime of h of x, times h prime of x, that's just the chain rule right over there, and that would be equal to, what's the derivative with respect to x of x? Well that's just going to be equal to one. So now it's interesting.