(x-5). (x-6) = <0 -3 .(x-4)<0

`a ) `

`(x-5)(x-6) ≤ 0`

`⇔` $\left[\begin{matrix} \begin{cases} x-5≥0\\x-6≤0 \end{cases}\\ \begin{cases} x-5≤0\\x-6≥0 \end{cases}\end{matrix}\right.$

`⇔` $\left[\begin{matrix} \begin{cases} x≥5\\x≤6 \end{cases}\\ \begin{cases} x≤5\\x≥6 \end{cases}\end{matrix}\right.$

`⇔` $\left[\begin{matrix} 5≤x≤6\\ 6≤x≤5 (L)\end{matrix}\right.$

Vậy `5≤x≤6`

`-----------------------`

`b) `

`-3. (x-4) < 0`

`⇔ x-4 > 0`

`⇔ x> 4`

Vậy `x> 4`