∵a1+a2+a3=18 ∴18q=q(a1+a2+a3)=a2+a3+a4=-9
∴ q=-1/2
∵18=a1+a2+a3=a1(1+q+q^2)=a1×(1-1/2+1/4)=3/4×a1
∴ a1=24
∴Sn=a1(q^n-1)/(q-1)
limSn(n->∞)=a1/(1-q)=24/(1+1/2)=16