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在各项都为正数的等比数列{an}中,首项a1=3,前三项和为21,则a3+a4+...
在各项都为正数的等比数列{an}中,首项a1=3,前三项和为21,则a3+a4+a5=( )
A.33
B.72
C.84
D.189
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