等差数列的前n项和为，且9，3，成等比数列. 若=3，则= ( �

等差数列的前n项和为，且9，3，成等比数列. 若=3，则= ( )

A. 7 B. 8 C. 12 D. 16

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等差数列的前n项和为，且9，3，成等比数列. 若=3，则= ( �

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