根据以上规律：3X4=1/3(3X4X5-2X3X4)nX(n+1)=1/3[nX(n+1)X(n+2)-(n-1)XnX(n+1)]1X2+2X3+3X4+、、、、、、+nX(n+1）=1/3[1X2X3-0X1X2+2X3X4-1X2X3+3X4X5-2X3X4+...+nX(n+1)X(n+2)-(n-1)XnX(n+1)]=1/3[nX(n+1)X(n+2)-0X1X2]=n(n+1)(n+2)/3
