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    • 1x2=1/3x（1x2x3-0x1x2) 2x3=1/3x(2x3x4-1x2x3) 3x4=1/3x(3x4x5-2x3x4) ......

    计算：3x[1x2+2x3+3x4+...+n(n+1)] x等于乘号
    赶作业用的，急需

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    推荐答案   2013-09-21
    3*[1x2+2x3+3x4+4x5+……+n(n+1)]
    =3*[1²+2²+3²+...........+n²+(1+2+3+.....+n)]
    =3*[n(n+1)(2n+1)/6+n(n+1)/2]
    =3*[n(n+1)(n+2)/3]
    =n(n+1)(n+2)
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