Imagine the plot of the function y =. The difference is the same as the difference and is the increment of the function y = at the point x = when you increase the argument "x" by the value of . The difference is the same as the difference and is the increment of the same function y = at the point x = when you increase the argument "x" by the value of . Then your statement follows the fact that the function y = has lower grade at the point x = than at the point x = . Very visual proof.