Tìm x, y biết: x + y + 9 = xy - 7
$⇔x-xy+y=-16$
$⇔x.(1-y)-1+y=-16-1$
$⇔x.(1-y)-(1-y)=-17$
$⇔(x-1)(1-y)=-17=17.(-1)=(-1).17=-17.1=1.(-17)$
$TH1:(x-1)(1-y)=1.(-17)$
$⇔x=2;y=18$
$TH2:(x-1)(1-y)=-17.1$
$⇔x=-16;y=0$
$TH3:(x-1)(1-y)=(-1).17$
$⇔x=0;y=-16$
$TH4:(x-1)(1-y)=17.(-1)$
$⇔x=18;y=2$
Xin câu trả lời hay nhất :v