So I'm going to say that this is equal to uppercase f of x. So all fair and good. Uppercase f of x is a function, if you give me an x value that's between a and b, it'll tell you the area under lowercase f of t between a and x. Now, the cool part. The fundamental theorem of calculus. The fundamental theorem of calculus tells us, let me write this down, because this is a big deal. 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.