[tex]\frac{a}{b}=\frac{18}{27}=\frac{2}{3}[/tex]
Do ƯCLN(a, b)=13
[tex]\Rightarrow \left\{\begin{matrix} a=13x\\b=13y\\(x, y)=1 \end{matrix}\right. (x; y\in \mathbb{N}^{*})[/tex]
[tex]\Rightarrow \frac{a}{b}=\frac{x}{y}=\frac{2}{3}[/tex]
Do ƯCLN(x, y)=1[tex]\Rightarrow \left\{\begin{matrix} x=2\\y=3 \end{matrix}\right.[/tex]
[tex]\Rightarrow \left\{\begin{matrix} a=26\\b=39 \end{matrix}\right.[/tex]
[tex]\Rightarrow \frac{a}{b}=\frac{26}{39} [/tex]