tìm x và y a) xy-3x+7y= 21 b) x+3x-7y=21 c) xy+5y-2x=10 Giup mình bài này với.

Bạn tham khảo nha:

a) xy-3x+7y=21

⇔xy-3x+7y-21=0

⇔(xy-3x)+(7y-21)=0

⇔x(y-3)+7(y-3)=0

⇔(y-3)(x+7)=0

⇔\(\left[ \begin{array}{l}y-3=0\\x+7=0\end{array} \right.\)⇔\(\left[ \begin{array}{l}y=3\\x=-7\end{array} \right.\) 

Vậy x=-7; y=3

b) xy+3x-7y=21

⇔xy+3x-7y-21=0

⇔(xy+3x)-(7y+21)=0

⇔x(y+3)-7(y-3)=0

⇔(y+3)(x-7)=0

⇔\(\left[ \begin{array}{l}y+3=0\\x-7=0\end{array} \right.\)⇔\(\left[ \begin{array}{l}y=-3\\x=7\end{array} \right.\) 

Vậy x=7; y=-3

c) xy+5y-2x=10

⇔xy+5y-2x=10

⇔xy+5y-2x-10=0

⇔(xy+5y)-(2x+10)=0

⇔y(x+5)-2(x+5)=0

⇔(x+5)(y-2)=0

⇔\(\left[ \begin{array}{l}x+5=0\\y-2=0\end{array} \right.\)⇔\(\left[ \begin{array}{l}x=-5\\y=2\end{array} \right.\) 

Vậy x=-5; y=2.