a1^2q^4=1,a1q^2=1S3=a1(1+q+q^2)=131+q+q^2=13q^212q^2-q-1=0q=1/3或q=-1/4(舍去)a1=9an=(1/3)^(n-3)S10=log3{1/3^[10(-4+7)/2]}=-25DDDDDDDDD
