`x . ( x + 3 ) = 0`
`⇔` \(\left[ \begin{array}{l}x = 0\\x + 3 = 0\end{array} \right.\)
`⇔` \(\left[ \begin{array}{l}x = 0\\x = - 3\end{array} \right.\)
Vậy `, x ∈ { 0 ; - 3 } .`
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`x - 2 . ( 2x - 10 ) = 0`
`⇔ x - 4x - 20 = 0`
`⇔ x . ( 1 - 4 ) = 0 + 20`
`⇔ x . ( - 3 ) = 20`
`⇔ x = 20 : ( - 3 )`
`⇔ x = ( - 20 )/3`
Vậy `, x = ( - 20 )/3 .`
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`( x - 1 ) . ( x^2 + 1 ) = 0`
`⇔` \(\left[ \begin{array}{l}x - 1 = 0\\x^2 + 1 = 0\end{array} \right.\)
`⇔` \(\left[ \begin{array}{l}x = 1\\x^2 = - 1 ( Loại )\end{array} \right.\)
Vậy `, x = 1 .`