Đáp án:
$ a) (x-4)(x+5) < 0$
$\text{Th1 :}$
⇔\(\left[ \begin{array}{l}x-4<0\\x+5>0\end{array} \right.\)
⇔\(\left[ \begin{array}{l}x<4(nhận)\\x>-5(nhận)\end{array} \right.\)
$\text{TH2 :}$
⇔\(\left[ \begin{array}{l}x-4>0\\x+5<0\end{array} \right.\)
⇔\(\left[ \begin{array}{l}x>4(loại)\\x<-5(loại)\end{array} \right.\)
$\text{Vậy -5<x<4}$
$b) (x+1,5)(x-3)>0$
$\text{TH1:}$
⇔\(\left[ \begin{array}{l}x+1,5>0\\x-3<0\end{array} \right.\)
⇔\(\left[ \begin{array}{l}x>-1,5(loại)\\x<3(loại)\end{array} \right.\)
$\text{TH2:}$
⇔\(\left[ \begin{array}{l}x+1,5<0\\x-3>0\end{array} \right.\)
⇔\(\left[ \begin{array}{l}x<-1,5(nhận)\\x>3(nhận)\end{array} \right.\)
$\text{Vậy 3<x<-1,5}$
