等差数列{an}的公差d≠0,数列{bn}是等比数列,若a1=b1,a3=b3,a7=b5,则b11=( ) A.a63 B.a36 C.a31 D.a13
问题描述:
等差数列{an}的公差d≠0,数列{bn}是等比数列,若a1=b1,a3=b3,a7=b5,则b11=( )
A. a63
B. a36
C. a31
D. a13
答
∵数列{bn}是等比数列
∴b32=b1•b5
又∵a1=b1,a3=b3,a7=b5
∴(b1+2d)2=b1•(b1+6d)
∴b1=2d
∴b11=b1•q10=a1•(
)2=32a1=a1+d(63−1)
b1+2d b1
∴b11=a63