a) `(x-2)(x-1)=0`
\(⇒\left[ \begin{array}{l}x-2=0\\x-1=0\end{array} \right.\)
\(⇒\left[ \begin{array}{l}x=2\\x=1\end{array} \right.\)
b) `(x^2+7)(x^2-49)<0`
Do `x^2+7≥7>0`
`⇒x^2-49<0`
`⇒(x-7)(x+7)<0`
Do `x-7<x+7`
$⇒\begin{cases}x-7<0\\x+7>0\end{cases}$
$⇒\begin{cases}x<7\\x>-7\end{cases}$
`⇒-7<x<7`
c) `(x^2-7)(x^2-49)<0`
Do `x^2-7>x^2-49`
$⇒\begin{cases}x^2-49<0\\x^2-7>0\end{cases}$
$⇒\begin{cases}x<7\\x>2\end{cases}$
`⇒2<x<7`
d) `(x^2+5)(x^2-25)=0`
Do `x^2+5≥5>0`
`⇒x^2-25=0`
`⇒(x-5)(x+5)=0`
\(⇒\left[ \begin{array}{l}x-5=0\\x+5=0\end{array} \right.\)
\(⇒\left[ \begin{array}{l}x=5\\x=-5\end{array} \right.\)
