What is the proof of of (N–1) + (N–2) + (N–3) + … + 1= N*(N–1)/2 [closed]
(N-1) + (N-2) +…+ 2 + 1 is a sum of N-1 items. Now reorder the items so, that after the first comes the last, then the second, then the second to last, i.e. (N-1) + 1 + (N-2) + 2 +… The way the items are ordered now you can see that each of … Read more