When you're taking the definite integral, one way of thinking, especially if you're taking a definite integral between a lower boundary and an x, one way to think about it is you're essentially taking an antiderivative. So we now see a connection. This is why it is the fundamental theorem of calculus. It connects differential calculus and integral calculus. Connection between derivatives, or maybe I should say antiderivatives, and integration, which before this video we just viewed integration as area under curve. Now that we see it has a connection to derivatives. Well, how would you actually use the fundamental theorem of calculus?