Fundamental theorem of calculus tells us that if we were to take the derivative of our capital F, so the derivative, let me make sure I have enough space here. So if I were to take the derivative of capital F with respect to x, which is the same thing as taking the derivative of this with respect to x, which is equal to the derivative of all of this business. Let me copy this. So copy and then paste. Which is the same thing. I've defined capital F as this stuff. So if I'm taking the derivative of the left-hand side, the same thing as taking the derivative of the right-hand side, the fundamental theorem of calculus tells us that this is going to be equal to f, lowercase f of x.