The fundamental theorem of calculus is a way of linking two different types of math problems together. It lets us take a problem involving one type of math – called integration – and find the same answer with a problem involving a different type of math – called differentiation. Integration has to do with finding the total area under a curve, like the area of a circle or the area between two points on a graph. Differentiation has to do with finding the change in a value over time, like how fast something is moving at different points in time. The fundamental theorem of calculus lets us mix these two types of problems together.