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. So let me write it this way. So if I had the integral from a to x of f prime of u du, well, what is this going to be equal to? Well, this is going to be, by the same logic, the antiderivative of this evaluated at x, so f of x minus f of a.