设首项为a1,公差为ds6/s12=3/10,s6/s12=(6a1+15d)/(12a1+66d)=3(2a1+5d)/6(2a1+11d)=(2a1+5d)/2(2a1+11d)则(2a1+5d)/2(2a1+11d)=3/10解得a1=2d则s3/s9=(3a1+3d)/(9a1+36d)=(6d+3d)/(18d+36d)=9/54=1/6