计算：1/1x2+1/2x3+1/3x4+1/4x5+……1/39x40

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答案和解析

1/1x2+1/2x3+1/3x4+1/4x5+……1/39x40
=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+……+1/39-1/40
=1-1/40
=39/40

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