`a, x(x - 3) = 0`
`⇔` \(\left[ \begin{array}{l}x=0\\x-3=0\end{array} \right.\) `⇒` \(\left[ \begin{array}{l}x=0\\x=3\end{array} \right.\)
`b, (x + 7)(x - 8) = 0`
`⇔` \(\left[ \begin{array}{l}x+7=0\\x-8=0\end{array} \right.\) `⇒` \(\left[ \begin{array}{l}x=-7\\x=8\end{array} \right.\)
`c, x(x - 3) < 0`
`⇔` $\left[\begin{array} {l} \begin{cases}x<0\\x-3>0\end{cases} \\\begin{cases}x>0\\x-3<0\end{cases} \end{array} \right.$ `⇔` $\left[\begin{array} {l} \begin{cases}x<0\\x>3\end{cases} \\\begin{cases}x>0\\x<3\end{cases} \end{array} \right.$ `⇔` \(\left[ \begin{array}{l}x∈∅\\0<x<3\end{array} \right.\)
`d, x(x + 3) ≥ 0`
`⇔` $\left[\begin{array} {l} \begin{cases}x≤0\\x+3≤0\end{cases} \\\begin{cases}x≥0\\x+3≥0\end{cases} \end{array} \right.$ `⇔` $\left[\begin{array} {l} \begin{cases}x≤0\\x≤-3\end{cases} \\\begin{cases}x≥0\\x≥-3\end{cases} \end{array} \right.$ `⇔` \(\left[ \begin{array}{l}x≤-3\\x≥0\end{array} \right.\)
`e, (x + 2)(x + 5) ≤ 0`
`⇔` $\left[\begin{array} {l} \begin{cases}x+2≤0\\x+5≥0\end{cases} \\\begin{cases}x+2≥0\\x+5≤0\end{cases} \end{array} \right.$ `⇔` $\left[\begin{array} {l} \begin{cases}x≤-2\\x≥-5\end{cases} \\\begin{cases}x≥-2\\x≤-5\end{cases} \end{array} \right.$ `⇔` \(\left[ \begin{array}{l}-5≤x≤-2\\x∈∅\end{array} \right.\)
`f, (x + 2)(x + 5) > 0`
`⇔` $\left[\begin{array} {l} \begin{cases}x+2<0\\x+5<0\end{cases} \\\begin{cases}x+2>0\\x+5>0\end{cases} \end{array} \right.$ `⇔` $\left[\begin{array} {l} \begin{cases}x<-2\\x<-5\end{cases} \\\begin{cases}x>-2\\x>-5\end{cases} \end{array} \right.$ `⇔` \(\left[ \begin{array}{l}x>-2\\x<-5\end{array} \right.\)
