3n*(n+1) = n(n+1)(n+2) - (n-1)n(n+1)
所以3S = (n+1)(n+2)(n+3) - 0
S = (n+1)(n+2)(n+3) /3具体过程呃。。。3n*(n+1) = n(n+1)(n+2) - (n-1)n(n+1)知道吧。设S=所要求的，那么3S = -0*1*2 + 1*2*3 - 1*2*3 + 2*3*4.... -n(n+1)(n+2) + (n+1)(n+2)(n+3)= (n+1)(n+2)(n+3)所以S = (n+1)(n+2)(n+3)/3