请在这里概述您的问题等比数列中,a1+a2+a3=18,a2+a3+a4=-9,且Sn=a1+2a2+3(a3+a4+…+an),则limSn=

设公比为q,则：a1+a2+a3=a1(1+q+q^2)=18 a2+a3+a4=a1q(1+q+q^2)=-9后式除以前式：q=-1/2a1=18/(1-1/2+1/4)=24Sn=a1+2a2+3(a3+a4+…+an)=24+2*24*(-1/2)+3*24*(-1/2)^2[1-1/2+1/2^2+……+(-1/2)^(n-1)]...