Well, if we took the integral from a to b of f prime of x dx, what is this going to be equal to? Well, this is going to be equal to the antiderivative of f prime of x, which is f of x, and you're going to evaluate that at b and a and subtract the difference. So this is going to be f of b minus f of a. This is straight out of the fundamental theorem of calculus. All right, so this is interesting. Well, what is it, instead of having a b, what if we had an x there? And if we're going to put an x as one of our bounds of integration, well, we'll use a different variable for our integration here.