Đáp án:
\(x \in \left\{ {1;\,\,43} \right\}\)
Giải thích các bước giải:
\(\begin{array}{l}
x \in N \Rightarrow x + 6 \ge 6\\
x + 6\,\,\,la\,\,\,uoc\,\,\,5x + 79\\
\Rightarrow 5x + 79\,\,\,chia\,\,het\,\,cho\,\,x + 6\\
Ta\,\,co:\,\,\,x + 6\,\,\,chia\,\,het\,\,cho\,\,\,x + 6\\
\Rightarrow 5\left( {x + 6} \right)\,\,\,chia\,\,het\,\,\,cho\,\,\,x + 6\\
\Rightarrow 5x + 30\,\,\,chia\,\,\,het\,\,cho\,\,x + 6\\
\Rightarrow \left( {5x + 79 - 5x - 30} \right)\,\,\,chia\,\,\,het\,\,cho\,\,x + 6\\
\Rightarrow 49\,\,\,\,chia\,\,\,het\,\,cho\,\,x + 6\\
Ma\,\,\,49\,\,chia\,\,het\,\,cho\,\,1,\,\,7,\,\,49\\
Lai\,\,co\,\,\,x + 6 \ge 6\\
\Rightarrow \left[ \begin{array}{l}
x + 6 = 7\\
x + 6 = 49
\end{array} \right. \Leftrightarrow \left[ \begin{array}{l}
x = 1\\
x = 43
\end{array} \right..\\
Vay\,\,\,x \in \left\{ {1;\,\,43} \right\}.
\end{array}\)
