[tex]\begin{cases}x+y=2x-4y\ |-y-2x\\x+y=7-y\end{cases}\\\begin{cases}-x=-5y\ |*(-1)\\x+y=7-y\end{cases}\\\begin{cases}x=5y\\x+y=7-y\ \leftarrow\text{podstawiam tu }x=5y\end{cases}\\(5y)+y=7-y\\6y=7-y\ |+y\\7y=7\\y=1\\\text{a teraz obliczam }x\text{ ze wzoru } x=5y\\x=5y\\x=5*1\\x=5\\\begin{cases}x=5\\y=1\end{cases}[/tex]
