a)Vì \(\left(x+1\right)\left(x-2\right)>0\)
Suy ra \(x+1;x-2\) cùng dấu
Xét \(\left\{{}\begin{matrix}x+1>0\\x-2>0\end{matrix}\right.\)\(\Rightarrow\left\{{}\begin{matrix}x>-1\\x>2\end{matrix}\right.\)
Xét \(\left\{{}\begin{matrix}x+1< 0\\x-2< 0\end{matrix}\right.\)\(\Rightarrow\left\{{}\begin{matrix}x< -1\\x< 2\end{matrix}\right.\)
b)\(\dfrac{x+1}{x-3}< 0\)
Suy ra \(x+1;x-3\) ngược dấu
Mà \(x+1>x-3\forall x\)
\(\Rightarrow\left\{{}\begin{matrix}x+1>0\\x-3< 0\end{matrix}\right.\)\(\Rightarrow\left\{{}\begin{matrix}x>-1\\x< 3\end{matrix}\right.\)