{an}为等比数列,且a1a9=64a3+a7=20,求a11

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{an}为等比数列,且a1a9=64a3+a7=20,求a11{an}为等比数列,且a1a9=64a3+a7=20,求a11{an}为等比数列,且a1a9=64a3+a7=20,求a11在等比数列{a

{an}为等比数列,且a1a9=64a3+a7=20,求a11
{an}为等比数列,且a1a9=64a3+a7=20,求a11

{an}为等比数列,且a1a9=64a3+a7=20,求a11
在等比数列{an}中,因a1a9=a3a7,所以a3a7=64,又a3+a7=20,
所以a3,a7是方程x^2-20x+64=0的两根,所以a3=4,a7=16(或反之)
所以a1=2,q^2=2(或a1=32,q^2=1/2)
所以a11=a1*q^10=2*2^5=64(或a11=a1*q^10=32*(1/2)^5=1)