设首项为a1,公差为d,依题意得a2^2=a1*a4=(a1+d)^2=a1*(a1+3d)化简,得(a1-d)*d=0当d≠0时,a1-d=0,则a1=da4=4d,a6=6d,a9=9d因为a6^2=a4*a9,所以此三项为等比数列当d=0时,此等差数列为常数列,an=a1所以a4=a1,a6=a1,a9=a...
