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已知等比數列{an}滿足a1=3,a1+a3+a5=21,則a3+a5+a7=           (  ) ...
問題詳情:已知等比數列{an}滿足a1=3,a1+a3+a5=21,則a3+a5+a7=           (  )                                    A.21          B.42         ...
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