Now I know what some of you are thinking because it's exactly what I would be thinking if someone just sprung this on me, is where does this come from? And I would tell you this comes straight out of the chain rule. We know that if a function and its inverse, we know that if we have a function and its inverse, that f of the inverse of our function, so f of h of x, f of h of x, we know that this is going to be equal to x. This literally, this comes out of them being each other's inverses. We could have also said h of f of x will also be equal to x. Remember, f is going to map, or h is going to map from some x to h of x, and then f is going to map back to that original x. That's what inverses do.