a) xy-3x=-19
x(y-3)=-19 = 1(-19) =-1.19
TH1 : $\left \{ {{x=1} \atop {y-3=-19}} \right.$ ⇒$\left \{ {{x=1} \atop {x=-16}} \right.$ (t/m)
TH2: $\left \{ {{x=-1} \atop {y-3=19}} \right.$ ⇒$\left \{ {{x=-1} \atop {y=22}} \right.$ (t/m)
TH3: $\left \{ {{x=-19} \atop {y-3=1}} \right.$ ⇒$\left \{ {{x=-19} \atop {y=4}} \right.$ (t/m)
TH4: $\left \{ {{x=19} \atop {y-3=-1}} \right.$ ⇒$\left \{ {{x=19} \atop {y=2}} \right.$ (t/m)
Vậy: $\left \{ {{x=1} \atop {x=-16}} \right.$ hoặc $\left \{ {{x=-1} \atop {y=22}} \right.$ hoặc $\left \{ {{x=-19} \atop {y=4}} \right.$ hoặc $\left \{ {{x=19} \atop {y=2}} \right.$
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b) 3x+4y-xy=16
⇒3x-xy+4y-12=16-12
⇒x(3-y)+4(y-3)=4
⇒x(3-y)-4(3-y)=4
⇒(x-4)(3-y)=4 = 1.4=2.2=(-2)(-2)=(-1)(-4)
Trình bày như trên.
