Question 1055154
{{{a[1]=a}}}
{{{a[2]=a*r}}}
{{{a[3]=a*r^2}}}
{{{a[4]=a*r^3}}}
So then,
{{{a[1]+a[3]=a+ar^2=6}}}
{{{a(1+r^2)=6}}}
and
{{{a[3]+a[4]=a^r^2+ar^3=-12}}}
{{{ar^2(1+r)=-12}}}
I graphed these two equations substituting (x,y) for (a,r) and found an intersection point.
{{{a=0.535}}}
{{{r=-3.196}}}
So then,
{{{S=a((1-r^N)/(1-r))}}}
{{{S=0.535((1-(-3.196)^20)/(1-(-3.196)))}}}
{{{S=-1574882205}}}