公比为4你先设a2,a3,a7为等比数列,a , a*q , a*q*q(q是公比)然后a*q-a=d(d是公差) a*q*q-a*q=4d然后下面比上面a消掉了,,q=4,懂了吧
设a2=a1+d a3=a1+2d a7=a1+6d由于 (a3)^2=a2*a7 可得a1=-2d/3a2=d/3 a3=4d/3公比q=a3/a2=4
