`a)`
`x^4 - 5x^2 + 4 = 0`
`⇔ x^4 - x^2 - 4x^2 + 4 = 0`
`⇔ x^2(x^2 - 1) - 4(x^2 - 1) = 0`
`⇔ (x^2 - 4)(x^2 - 1) = 0`
`⇒` \(\left[ \begin{array}{l}x=±2\\x=±1\end{array} \right.\)
`Vậy S = {± 2 ; ± 1}`
`b)`
`(x^2 + x)^2 - 3x^2 - 3x - 4 = 0`
`⇔ [x(x + 1)]^2 - 3x(x + 1) - 4 = 0`
`⇔ x^2(x + 1)^2 - 3x(x + 1) - 4 = 0`
`⇔ x^2(x + 1)^2 + x(x + 1) - 4x(x + 1) - 4 = 0`
`⇔ x(x + 1)[x(x + 1) + 1] - 4[x(x + 1) + 1] = 0`
`⇔ [x(x + 1) + 1][x(x + 1) - 4] = 0`
`⇔ (x^2 + x + 1)(x^2 + x - 4) = 0`
`Do x^2 + x + 1 > 0`
`⇒ x^2 + x - 4 = 0`
`⇒ x^2 + 2 . x . 1/2 + 1/4 = 17/4`
`⇔ (x + 1/2)^2 = 17/4`
`⇒ x + 1/2 = \sqrt{17}/2 ; x + 1/2 = -\sqrt{17}/2`
`⇔ x = (\sqrt{17} - 1)/2 ; x = (-\sqrt{17} - 1)/2`
`Vậy S = {(\sqrt{17} - 1)/2 ; (-\sqrt{17} - 1)/2}`
`c)`
`(x + 1)^4 + x^2 + 2x - 1 = 0`
`⇔ (x + 1)^2(x + 1)^2 + x^2 + 2x - 1 = 0`
`⇔ (x^2 + 2x + 1)(x^2 + 2x + 1) + x^2 + 2x - 1 = 0`
`Đặt x^2 + 2x = a `
`⇒ (a + 1)(a + 1) + a - 1 = 0`
`⇒ a^2 + 2a + 1 + a - 1 = 0`
`⇔ a^2 + 3a = 0`
`⇔ a(a + 3) = 0`
`⇒` \(\left[ \begin{array}{l}a = 0\\a = -3\end{array} \right.\)
`Với a = 0`
`⇒ x^2 + 2x = 0`
`⇒ x(x + 2) = 0`
`⇒` \(\left[ \begin{array}{l}x = 0\\x = -2\end{array} \right.\)
`Với a = -3`
`⇒ x^2 + 2x = -3`
`⇒ x^2 + 2x + 3 = 0`
`⇔ x^2 + 2x + 1 + 2 = 0`
`⇒ (x + 1)^2 + 2 = 0` `text((vô lí))`
`Vậy S = {0 ; -2}`
