để \(\dfrac{x^2\left(x-3\right)}{x-9}< 0\) thì \(x^2\left(x-3\right)\:v\text{à}\:x-9\:ph\text{ải}\:kh\text{ác}\:nhau\)
\(\Rightarrow\left[{}\begin{matrix}\left\{{}\begin{matrix}x^2\left(x-3\right)>0\\x-9< 0\end{matrix}\right.\\\left\{{}\begin{matrix}x^2\left(x-3\right)< 0\\x-9>0\end{matrix}\right.\end{matrix}\right.\Rightarrow\left[{}\begin{matrix}\left\{{}\begin{matrix}x^3>3x^2\\x< 9\end{matrix}\right.\\\left\{{}\begin{matrix}x^3< 3x^2\\x>9\end{matrix}\right.\end{matrix}\right.\\ \Rightarrow\left[{}\begin{matrix}\left\{{}\begin{matrix}x>3\\x< 9\end{matrix}\right.\\\left\{{}\begin{matrix}x< 3\\x>9\end{matrix}\right.\end{matrix}\right.\Rightarrow3< x< 9\)
