But I'm going to call this thing, which we're saying it equals the slope of the tangent line, and it does equal the slope of the tangent line, I'm going to call this the derivative of f. Let me write that down. The derivative of f. And I'm going to say that this is equal to f prime of x. And this is going to be another function. Because remember, the slope changes at every x value. No matter what x value you pick, the slope is going to be different. It doesn't have to be, but the way I drew this curve, it is different. It can be different.