s3=a1(1-q^3)/(1-q)s9=a1(1-q^9)/(1-q)s6=a1(1-q^6)/(1-q)s6-s9=s9-s3,代入得：2q^9=q^3+q^6即：2q^7=q+q^4若a2,a8,a5成等差数列那么a5-a8=a8-a2,a2+a5=2a8即：a1q+a1q^4=2a1q^7因为2q^7=q+q^4所以a2,a8,a5成等差数列
