a3=根(a2*a4)=1 a1+a2+a3=1/q2+1/q+1=13, q=-1/4(舍）,或q=1/3an=27*(1/3)^nbn=log3 an=log3 27+ log3 (1/3)^n=3-nSbn=30-(1+10)*10/2=-25
