1+2+...+55 = 55*56/2 = 55*28 =
1+2+...+69 = 69*70/2 = 69*35 =inmultiri simple stii sa faci tu
1+2+...+32 = 32*33/2 = 16*33 =
1+2+...+45 = 45*46/2 = 45*23 =
1+2+...+78 = 78*79/2 = 39*79 =
1+2+...+ 90 = 90*91/2 = 45*91 =
1+2+...+ 54 = 54*(54+1)/2 = 27*55 =
1+2+3+...+n(n+1)/2 = n(n+1)/2 * (1+n(n+1)/2) / 2 = n(n+1)/4 * (n^2 + n +2) = n(n+1)(n^2 + n + 2)/8
