This is how we evaluate definite integrals. Now, let's think about what the negative of this is. So the negative of that, of a to b of f of t dt, is just going to be equal to the negative of this, which is equal to the negative of F of b minus F of a, which is equal to capital F of a minus capital F of b. All I did is distribute the negative sign and then switch the two terms. But this right over here is equal to the definite integral from, instead of a to b, but from b to a of F of t dt. So notice, when you put a negative, that's just like switching the signs or switching the boundaries. Or if you switch the boundaries, they're the negatives of each other.