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[主观题]
若-1,以,6,c,-9五个数成等比数列,则()
A.b=3,ac=9
B.b=-3,ac=9
C.b=-3,ac=-9
D.b=3,ac=-9
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A.b=3,ac=9
B.b=-3,ac=9
C.b=-3,ac=-9
D.b=3,ac=-9
在3与9之间插入两个正数,使前三个数成等比数列,后三个数成等差数列,则这两个正数的和为()
A.27/2
B.45/4
C.21/2
D.19/2
已知等比数列{an}中,a1=16,公比q=(1/2)
(Ⅰ)求数列{an}的通项公式;
(Ⅱ)若数列{an}的前n项的和Sn=124,求n的值