[tex]\text{The common ratio of a geometric sequence}\ a_n:\\\\r=\dfrac{a_{n+1}}{a_n}\\\\\text{We divide next term by the previous one.}\\\\r=\dfrac{9}{27}=\dfrac{9:9}{27:9}=\dfrac{1}{3}\\\\r=\dfrac{3}{9}=\dfrac{3:3}{9:3}=\dfrac{1}{3}\\\\r=\dfrac{1}{3}\\\vdots[/tex]
