Gọi \(d=\left(2n+9;n+5\right)\)
\(\left\{{}\begin{matrix}2n+9⋮d\\n+5⋮d\end{matrix}\right.\)\(\Rightarrow\left\{{}\begin{matrix}2n+9⋮d\\2n+10⋮d\end{matrix}\right.\)
\(\Rightarrow\left(2n+10\right)-\left(2n+9\right)⋮d\)
\(\Rightarrow1⋮d\Rightarrow d=1\)
=> 2n+9 và n+5 nguyên tố cùng nhau