令{an}等比数列的公比为q,q>0.
则a3*a9=a1^2*q^10=a6^2
则a3*a9=a6^2=2a5^2,a6^2/a5^2=2,则q=a6/a5=根号2
则a1=a2/q=1/根号2=根号2/2