$\frac{7n+4}{5n+3}$
Gọi $d$ là ƯCLN $\left( 7n+4;\,\,5n+3 \right)$
$\Rightarrow \left\{ \begin{align} & 7n+4\,\,\vdots \,\,d \\ & 5n+3\,\,\vdots \,\,d \\ \end{align} \right.$
$\Rightarrow \left\{ \begin{align} & 35n+20\,\,\vdots \,\,d \\ & 35n+21\,\,\vdots \,\,d \\ \end{align} \right.$
$\Rightarrow \,1\,\,\vdots \,\,d$
$\Rightarrow d=\left\{ \pm 1 \right\}$
Vậy $\frac{7n+4}{5n+3}$ là phân số tối giản