1x2+2x3+3x4+……+（n-1）n=1/3×(n-1)n(n+1)1x2+2x3+3x4+……+10x11=1/3×10×11×12=440
解：--------------------------1x2+2x3+3x4+……+（n-1）n=1/3×(n-1)n(n+1)1x2+2x3+3x4+……+10x11=1/3×10×11×12=440
1/3x(n-1)xnx(n+1)440
