At some point, your instantaneous slope is going to be the same as the average slope. Now how would we write that mathematically? Well let's calculate, let's calculate, well let's calculate the average slope over this interval. Well the average slope over this interval, or the average change, the slope of the secant line, is going to be our change in y, our change in y right over here, over our change in x, over our change in x. Well what is our change in y? Our change in y is f of b minus f of a, minus f of a, and that's going to be over, that is going to be over our change in x. Over b minus, b minus a.