`a) (x-5)(x-7)>0`
`<=>`\(\left[ \begin{array}{l}\begin{cases}x-5>0\\x-7>0\end{cases}\\\begin{cases}x-5<0\\x-7<0\end{cases}\end{array} \right.\)
`<=>`\(\left[ \begin{array}{l}\begin{cases}x>5\\x>7\end{cases}\\\begin{cases}x<5\\x<7\end{cases}\end{array} \right.\)
`<=>`\(\left[ \begin{array}{l}x>7\\x<5\end{array} \right.\)
Vậy `x>7` hoặc `x<5`
`b) (x+4)(x-3)<0`
Do `x+4>x-3`
`<=> x+4>0>x-3`
`<=> -4<x<3`
Vậy `-4<x<3`
`c) (5-x)(x-8)>0`
`<=> (x-5)(x-8)<0`
Do `x-5>x-8`
`<=> x-5>0>x-8`
`<=> 5<x<8`
Vậy `5<x<8`
