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. Now, why is this a big deal? Why does it get such an important title as the fundamental theorem of calculus? Well, it tells us that for any continuous function f, if I define a function that is the area under the curve between a and x right over here, that the derivative of that function is going to be f. So let me make it clear.