2 1, sum of top n items in arithmetic progression:1+2+...+n = n (n+1)/2,1+2+...+(n-1) = n (.

The next step is the same as 22.

22, you will get a > 0 first. Because if a < 0, the whole limit looks like "+∞" +∞ ",so the limit is +∞ and can't be 0. Only when a > 0, the form of limit is "+∞"-"+∞", and the simplified limit may be 0.