设a1=1,(an+1一an)的和为_________.

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问题 设a1=1,(an+1一an)的和为_________.

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答案2020

解析 级数(an+1一an)的部分和数列为
    Sn=(a2一a1)+(a3一a2)+…+(an+1—an)
    =an+1一a1=an+1—1.
=2021—1=2020.
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