1x2x3+2x3x4+3x4x5+.+n(n+1)(n+2)
问题描述：
1x2x3+2x3x4+3x4x5+.+n(n+1)(n+2)
等于多少.
1X2+2X3+3X4+....+n(n+1)=1/3Xn(n+1)(n+2)
答
1+2+...+n=n(n+1)/2
1^2+2^2+…+n^2=n(n+1)(2n+1)/6
1^3+2^3+…+n^3=n^2 * (n+1)^2 / 4
明确这三个式子就好了.