(13分)已知数列的前n项和为,,,等差数列中,且,又、、成等比数列. (Ⅰ)求数列、的通项公式; (Ⅱ)求数列的前n项和. 答案:(14分) 解:(Ⅰ)∵,, ∴, ∴, ∴, ∴ …………………………2分 而,∴ ∴数列是以1为首项,3为公比的等比数列, ∴ …………………………4分 ∴, 在等差数列中,∵,∴。 又因、、成等比数列,设等差数列的公差为d, ∴()
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