But now I want to do something a little bit more interesting. I want to find the derivative with respect to x, not just of f of x, but the product of two functions, f of x times g of x. And if I can come up with a simple thing for this, that essentially is the product rule. Well, if we just apply the definition of a derivative, that means I'm going to take the limit as h approaches zero. In the denominator, I'm going to have an h. In the denominator, I'm going to write a big, it's going to be a big rational expression. In the denominator, I'm going to have an h. And then I'm going to evaluate this thing at x plus h. So that's going to be f of x plus h, g of x plus h. And from that, I'm going to subtract this thing, evaluate it at f of x. Or, sorry, this thing evaluate it at x.