设等差数列｛an｝的公差为d不为0,a1＝9d,若ak是a1与a2k的等比中项,则k＝

a1=9d ak=a1+(k-1)*d=9d+(k-1)*d a2k=a1+(2k-1)*d=9d+(2k-1)*d ak^2=a1*a2k 化简后可求出k=4