Think about this expression. Remember, this expression, all it's doing is calculating the slope between the point X comma F of X and the point C comma F of C. So if X is say out here, this is X comma F of X, it's going to be calculated, and so as we take the limit as X approaches C from the left, we'll be looking at this slope. And then as we get closer, we'll be looking at this slope, which is actually going to be the same. In this case it would be a negative one. So as X approaches C from the left, this expression would be negative one. But as X approaches C from the right, this expression is going to be one. The slope of the line that connects these points is one.