S=1x2x3x4+5*2x3x4+.(n+3)n(n+1)(n+2)S= 1x2x3x4+.(n-1)n(n+1)(n+2)+n(n+1)(n+2)(n+3)移项这是我最初的想法,那么设M= 2x3x4+3x4x5+.n(n+1)(n+2）则（5-1）M +1x2x3x4 -n(n+1)(n+2)(n+3)=S-S=0这样子是算不出S的不过...