3xy + y=4-x
<=>9xy+3y=12-3x
<=>9xy+3y+3x+1=13
<=>3y.(3x+1)+(3x+1)=13
<=>(3x+1)(3y+1)=13
<=> *{3x+1=13y+1=13{3x+1=13y+1=13<=>{x=0y=4{x=0y=4(nhận)
*{3x+1=123y+1=1{3x+1=123y+1=1<=>{x=4y=0{x=4y=0(nhận)
*{3x+1=−13y+1=−13{3x+1=−13y+1=−13<=>{x=−23y=−143{x=−23y=−143(loại)
*{3x+1=−133y+1=−1{3x+1=−133y+1=−1<=>{x=−143y=−23{x=−143y=−23(loại)
Vậy x=4 thì y=0 ; x=0 thì y=4