Question 1024168
{{{a[1]=a}}}
{{{a[2]=a*r=9}}}
{{{a[3]=a*r^2}}}
{{{a[4]=a*r^3=81}}}
So then,
{{{(a*r^3)/(a*r)=81/9}}}
{{{r^2=9}}}
{{{r=3}}}
and
{{{a*3=9}}}
{{{a=3}}}
I think you want the sum of the first 4 terms(?),
{{{S=a[1]+a[2]+a[3]+a[4]=a(1+r+r^2+r^3)=3(1+3+9+27)=120}}}
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It could also be the case that,
{{{r=-3}}} and then,
{{{a*(-3)=9}}}
{{{a=-3}}}
So then,
{{{S=(-3)(1-3+9-27)}}}
{{{S=60}}}