a) $\left \{ {{2x-y=5} \atop {2x-3y=1}} \right.$
`<=>` $\left \{ {{2x-y=5} \atop {2y=4}} \right.$
`<=>` $\left \{ {{2x-y=5} \atop {y=2}} \right.$
`<=>` $\left \{ {{2x-2=5} \atop {y=2}} \right.$
`<=>` $\left \{ {{2x=7} \atop {y=2}} \right.$
`<=>` $\left \{ {{x=7/2} \atop {y=2}} \right.$
Vậy hệ pt có 1 nghiệm `(x;y)=(7/2;2)`
b) $\left \{ {{2x-3y=1} \atop {3x-y=5}} \right.$
`<=>` $\left \{ {{2x-3y=1} \atop {-9x+3y=-15}} \right.$
`<=>` $\left \{ {{2x-3y=1} \atop {-7x=-14}} \right.$
`<=>` $\left \{ {{2.2-3y=1} \atop {x=2}} \right.$
`<=>` $\left \{ {{-3y=-3} \atop {x=2}} \right.$
`<=>` $\left \{ {{y=1} \atop {x=2}} \right.$
Vậy hệ pt có nghiệm `(x;y)=(2;1)`
