Đáp án:
a) $\left \{ {{3x+2y=2} \atop {-x-4y=3}} \right.$
<=> $\left \{ {{6x+4y=4} \atop {-x-4y=3}} \right.$
<=> <=> $\left \{ {{6x+4y=4} \atop {5x=7}} \right.$
<=> <=> $\left \{ {{6x+4y=4} \atop {x=7/5}} \right.$
<=> <=> $\left \{ {{6.7/5+4y=4} \atop {x=7/5}} \right.$
<=> <=> $\left \{ {{y=-1,1} \atop {x=7/5}} \right.$
b) $\left \{ {{2x+y=5} \atop {x-y=1}} \right.$
<=> $\left \{ {{2x+y=5} \atop {3x=6}} \right.$ <=> $\left \{ {{2x+y=5} \atop {x=2}} \right.$
<=> $\left \{ {{2.2+y=5} \atop {x=2}} \right.$
<=> $\left \{ {{y=1} \atop {x=2}} \right.$
c) $\left \{ {{10x-9y=1} \atop {15x+21y=36}} \right.$
<=> $\left \{ {{30x-27y=3} \atop {30x+42y=72}} \right.$
<=> $\left \{ {{30x-27y=3} \atop {-69y=-69}} \right.$
<=> $\left \{ {{30x-27y=3} \atop {y=1}} \right.$
<=> $\left \{ {{30x-27.1=3} \atop {y=1}} \right.$
<=> $\left \{ {{x=1} \atop {y=1}} \right.$
