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在公差不为零的等差数列{an}中,又2a1-a7+2a11=0,数列{bn}是等比数列,且a7+1=b7,则b6b8=

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在公差不为零的等差数列{an}中,又2a1-a7+2a11=0,数列{bn}是等比数列,且a7+1=b7,则b6b8=
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答案和解析
a7 = a1 + 6d
a11 = a1 + 10d
2a1 - a7 + 2a11 = 2a1 - a1 - 6d + 2a1 + 20 d = 3a1 + 14d = 0
所以d = -3/14 a1
a7 + 1 = b7 = a1 + 6d + 1 = a1 - 18/14 a1 + 1 = 1 - 2a1/7
设 a1 = 1
b7 = 5/7
b6b8 = 25/49