Đáp án:
Giải thích các bước giải:
$|2x+1|=-x$ $(x≤0)$
$⇔$\(\left[ \begin{array}{l}2x+1=-x\\-2x-1=-x\end{array} \right.\)
$⇔$\(\left[ \begin{array}{l}3x=-1\\-x=1\end{array} \right.\)
$⇔$\(\left[ \begin{array}{l}x=\frac{-1}{3}(ktm)\\x=-1(tm)\end{array} \right.\)
Vậy $S=${$-1$}
$ $
$ $
$xy+y-x-4=0$
$⇔y.(x+1)-x-1-3=0$
$⇔y.(x+1)-(x+1)=3$
$⇔(y-1).(x+1)=3=3.1=1.3=(-3).(-1)=(-1).(-3)$
$TH1:(y-1).(x+1)=3.1$
$⇔y=4;x=0$
$TH2:(y-1).(x+1)=1.3$
$⇔y=2;x=2$
$TH3:(y-1).(x+1)=-3.(-1)$
$⇔y=-2;x=-2$
$TH4:(y-1).(x+1)=(-1).(-3)$
$⇔y=0;x=-4$
Mà $x,y∈N$
$⇔(x;y)∈${$(4;0);(2;2)$}
