\(\left(x-3\right)\left(x-5\right)< 0\)
\(\Rightarrow\left\{{}\begin{matrix}\left\{{}\begin{matrix}x-3< 0\\x-5>0\end{matrix}\right.\\\left\{{}\begin{matrix}x-3>0\\x-5< 0\end{matrix}\right.\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}\left\{{}\begin{matrix}x< 3\\x>5\end{matrix}\right.\\\left\{{}\begin{matrix}x>3\\x< 5\end{matrix}\right.\end{matrix}\right.\)
Vậy 3<x<5 là giá trị cần tìm
(x-3).(x-5)<0
\(\Rightarrow\) Sẽ có 1 số là số âm, mà (x-3)>(x-5)
\(\Rightarrow\) x-3>0
\(\Rightarrow\) x > 3
\(\Rightarrow\)x-5<0
\(\Rightarrow\)x<5
\(\Rightarrow\)3<x<5
\(\Rightarrow\) x=4
