Do you mean 'approaches infinity'? Because at n=0 the expression is maximal :-) But the point is not to drop small terms at the extremes - it is still an infin

Do you mean 'approaches infinity'? Because at n=0 the expression is maximal :-) But the point is not to drop small terms at the extremes - it is still an infinite sum. The point is to approximate sections of the function by rectangles, and those rectangles are a bad approximation around 0 too.