選択した画像 sum of 1/n^2 formula 109622-Sum of series 1+2+3+4+....+n formula

 The way the items are ordered now you can see that each of those pairs is equal to N (N11 is N, N22 is N) Since there are N1 items, there are (N1)/2 such pairs So you're adding N (N1)/2 times, so the total value is N*(N1)/2