Giải hệ phương trình: x + 2y + 6z = 5; -x + y - 2z = 3; x - 4y - 2z = 13

* Hướng dẫn giải

$\begin{array}{l}\left\{\begin{array}{l}x+2y+6z=5\\ -x+y-2z=3\\ x-4y-2z=13\end{array}\right.\iff \left\{\begin{array}{l}x+2y+6z=5\\ 3y+4z=8\\ x-4y-2z=13\end{array}\right.\iff \left\{\begin{array}{l}x+2y+6z=5\\ 3y+4z=8\\ 6y+8z=-8\end{array}\right.\\ \iff \left\{\begin{array}{l}x+2y+6z=5\\ 3y+4z=8\\ 3y+4z=-4\end{array}\right.\iff \left\{\begin{array}{l}x+2y+6z=5\\ 3y+4z=8\\ 0=12\end{array}\right..\end{array}$