(Assuming the limit is to 0+). Um, let . We must find a , s.t. if , . Since is increasing, . So chuse and it should follow...

Note that delta is a function of epsilon only. You can get amusing results if it is allowed to depend on x. http://math.berkeley.edu/~critch/Pi_equals_0,_or,_how_I_learned_to_stop_worrying_an d_let_delta_depend_on_x.pdf