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数学
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等差数列{a
n
}的前m项和为30,前2m项和为100,则它的前3m项和为______.
人气:233 ℃ 时间:2020-05-25 06:17:48
解答
等差数列{a
n
}的每m项的和成等差数列,设前3m项和为 x,则 30,100-30,x-100 成等差数列,
故 2×70=30+(x-100 ),x=210,
故答案为:210.
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