S3=a1(1-q^3)/（1-q),S9=a1(1-q^9)/(1-q).S6=a1(1-q^6)/(1-q）因为S3,S9,S6成等差数列，则S3+S6=2S9 即q^3+q^6=2*q^9 .同除以q^3,则2q^6-q^3-1=0 q^3=-1/2a2+a5=a2(1+q^3)=(1/2)a2=2* q^6*a2得am=a2* q^6=a8所以m=8望采纳
