xy + 3x - 7y = 21
xy - 7y + 3x - 21 = 0
y(x - 7) + 3(x - 7) = 0
(y + 3).(x - 7) = 0
<=> y + 3 = 0 hoặc x - 7 =0
<=> y = -3 hoặc x = 7
#Đức Lộc#
\(xy+3x-7y=21\)
\(x\left(y+3\right)-7y-21=21-21\)
\(x\left(y+3\right)-7\left(y+3\right)=0\)
\(\left(y+3\right)\left(x-7\right)=0\)
\(\Rightarrow\hept{\begin{cases}y+3=0\\x-7=0\end{cases}\Rightarrow\hept{\begin{cases}y=-3\\x=7\end{cases}}}\)
Vậy khi y = -3 thì x \(\in\)N
x = 7 thì y \(\in\)N
PP/ss: Hoq chắc
xy + 3x - 7y = 21
=> x(y + 3) - 7(y + 3) + 21 = 21
=> (x - 7)(y + 3) = 0
\(\Rightarrow\orbr{\begin{cases}x-7=0\\y+3=0\end{cases}\Rightarrow\orbr{\begin{cases}x=7\\y=-3\end{cases}}}\)
Vậy......................
\(xy+3x-7y=21\)
\(\Rightarrow xy+3x-7y-21=0\)
\(\Rightarrow x\left(y+3\right)-7\left(y+3\right)=0\)
\(\Rightarrow\left(x-7\right)\left(y+3\right)=0\)
\(\Leftrightarrow\orbr{\begin{cases}x-7=0\\y+3=0\end{cases}}\)
\(\Leftrightarrow\orbr{\begin{cases}x=7\\y=-3\end{cases}}\)
Vậy................................
