23题，谢谢，给好评
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1x2+2x3+3x4+……+10x11
=1/3x(1x2x3-0x1x2)+1/3x(2x3x4-1x2x3)+1/3x(3x4x5-2x3x4)+……+1/3x(10x11x12-9x10x11)
=1/3x(10x11x12)
=440
1x2+2x3+3x4+……+nx(n+1)
=1/3x(1x2x3-0x1x2)+1/3x(2x3x4-1x2x3)+1/3x(3x4x5-2x3x4)+……+1/3x(nx(n+1)x(n+2)-(n-1)xnx(n+1))
=1/3x(nx(n+1)x(n+2))
=1/3x(1x2x3-0x1x2)+1/3x(2x3x4-1x2x3)+1/3x(3x4x5-2x3x4)+……+1/3x(10x11x12-9x10x11)
=1/3x(10x11x12)
=440
1x2+2x3+3x4+……+nx(n+1)
=1/3x(1x2x3-0x1x2)+1/3x(2x3x4-1x2x3)+1/3x(3x4x5-2x3x4)+……+1/3x(nx(n+1)x(n+2)-(n-1)xnx(n+1))
=1/3x(nx(n+1)x(n+2))
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