`a) (x² - 2x)² - 6(x² - 2x) + 9 = 0 `
`⇔ (x² - 2x)² - 2.(x² - 2x).3 + 3² = 0`
`⇔ (x² - 2x - 3)² = 0`
`⇔ x² - 2x - 3 = 0`
`⇔ (x - 3)(x + 1) = 0`
\begin{cases}x-3=0\\x+1=0\\\end{cases}
\begin{cases}x=3\\x=-1\\\end{cases}
`2x(8x - 1)(8x² - x + 2) - 126 = 0`
`⇔ (8x² - x)(8x² - x - 7) + 9(8x² - x) - 63 = 0`
`⇔ (8x² - x - 7)(8x² - x + 9) = 0 `
`⇔ 8x² - x - 7 = 0 ( `vì `8x² - x + 9 > 0)`
`⇔ (x - 1)(8x + 7) = 0`
\begin{cases}x-1=0\\8x+7=0\\\end{cases}
\begin{cases}x=1\\x=-7/8\\\end{cases}
