`(x - 1)² - 9(x + 1)² = 0`
`(x - 1)² - 3²(x + 1)² = 0`
`(x - 1)² - [3(x + 1)]² = 0`
`(x - 1)² - (3x + 3)² = 0`
`(x - 1 + 3x + 3)(x - 1 - 3x - 3) = 0`
`(4x + 2)(-2x - 4) = 0`
\(\left[ \begin{array}{l}4x + 2 = 0\\-2x - 4 = 0\end{array} \right.\)
\(\left[ \begin{array}{l}x= \frac{-1}{2} \\x=-2\end{array} \right.\)
`Vậy x = { -1/2 ; -2 }`