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