an=a1q^(n-1)a4=a1q^396=-1.5*q^364=q^3q=-64开3次方也就是q=-4所以Sn=a1(1-q^n)/(1-q)S4=-1.5(1-256)/5=76.5
an=a1q^(n-1)a4=a1q^396=-1.5*q^3q=-4Sn=a1(1-q^n)/(1-q)S4=-1.5(1-256)/5=76.5
