摘要：10． 等差数列中.且成等比数列.求数列前20项的和． 解:设数列的公差为.则 . . ．····························································································· 3分 由成等比数列得. 即. 整理得. 解得或．····································································································· 7分 当时.．·················································································· 9分 当时.. 于是．······················································ 12分

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