The formula given
[tex]\begin{gathered} s=\frac{a_1}{1-r} \\ \text{where a}_1=\text{ first term and r= common ratio} \end{gathered}[/tex]Can be used to find the sum to infinity of a series as shown below
Step 1: Write out the sequence
9 + 3 + 1 ...
Step 2: Obtain the first term and common ratio
[tex]\begin{gathered} a_{1=}\text{first term=9} \\ \text{common ratio=r=}\frac{\sec ond\text{ term}}{first\text{ term}}=\frac{3}{9}=\frac{1}{3} \end{gathered}[/tex]Step 3: Substitute the value of the first term and common ratio into the equation
[tex]s=\frac{9}{1-\frac{1}{3}}[/tex][tex]s=\frac{9}{\frac{2}{3}}[/tex][tex]s=9\text{ x }\frac{3}{2}[/tex][tex]s=\frac{27}{2}[/tex][tex]s=13.5[/tex]Answer = 13.5