1x2x3+2x3x4+3x4x5+.+n(n+1)(n+2)
1x2x3+2x3x4+3x4x5+4x5x6+...+n(n+1)(n+2)=
1/(1x2x3)+1/(2x3x4)+1/(3x4x5)+.1/(nx(n+1)x(n+2)=?
求和1x2x3+2x3x4+...+n(n+1)(n+2)
1/1x2x3+1/2x3x4+1/3x4x5
1x2X3+2x3X4+3x4X5+…+7X8X9=?
1x2x3+2x3x4+3x4x5+…+8x9x10
1x2x3+2x3x4+3x4x5+...+7x8x9=,
1x2x3+2x3x4+3x4x5+.+10x11x12
1/1x2x3+1/2x3x4+1/3x4x5+1/4x5x6+.+1/48x49x50=
1/1x2x3+1/2x3x4+1/3x4x5+.+1/11x12x13=
1/1x2x3+1/2x3x4+1/3x4x5+.+1/9x10x11=
请1/1x2x3+1/2x3x4+1/3x4x5+1/4x5x6=