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等比数列{an}中,a1+a2=30,a3+a4=60,则a7+a8=(   )(A)120         ...
问题详情:等比数列{an}中,a1+a2=30,a3+a4=60,则a7+a8=(   )(A)120                           (B)180                    (C)240                    (D)2...
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