a1,a2,a3,成等比数列 即a2^2=a1a3 即( a1+d)^2=a1(a1+2d) a1^2+2a1d+d^2=a1^2+2a1d 得:d^2=0,d=0 所以,a1=a2=a3. 那么,公比q=a2/a1=1
