Bài 2:
\(xy-3x+7y=21\)
\(\Rightarrow x\left(y-3\right)+7y-21=0\)
\(\Rightarrow x\left(y-3\right)+7\left(y-3\right)=0\)
\(\Rightarrow\left(x+7\right)\left(y-3\right)=0\)
\(\Rightarrow\left\{\begin{matrix}x+7=0\\y-3=0\end{matrix}\right.\Rightarrow\left\{\begin{matrix}x=-7\\y=3\end{matrix}\right.\)
Vậy \(\left\{\begin{matrix}x=-7\\y=3\end{matrix}\right.\)
Bài 1:
\(1-2-3+4+5-6-7+...+97-98-99+100\)
\(=\left(1-2-3+4\right)+\left(5-6-7+8\right)+...+\left(97-98-99+100\right)\)
\(=0+0+0+...+0+0+0\)
\(=0\)