a, (3x - 5)(5 - 3x) + 9(x + 1)² = 30
⇔ - (3x - 5)(3x - 5) + 9(x + 1)² = 30
⇔ - (3x - 5)² + 9(x + 1)² = 30
⇔ [3(x + 1) - (3x - 5)] [3(x + 1) + (3x - 5)] = 30
⇔ (3x + 3 - 3x + 5) (3x + 3 + 3x - 5) = 30
⇔ 8(6x - 2) = 30
⇔ 6x - 2 = 30 : 8
⇔ 6x - 2 = $\dfrac{15}{4}$
⇔ 6x = $\dfrac{15}{4}$ + 2
⇔ 6x = $\dfrac{15+8}{4}$ = $\dfrac{23}{4}$
⇔ x = $\dfrac{23}{4}$ : 6 = $\dfrac{23}{24}$
Vậy x = $\dfrac{23}{24}$
b, (x + 4)² - (x - 1)(x + 1) = 16
⇔ x² + 8x + 16 - (x² - 1) = 16
⇔ x² + 8x + 16 - x² + 1 = 16
⇔ 8x + 17 = 16
⇔ 8x = 16 - 17
⇔ 8x = -1
⇔ x = $\dfrac{-1}{8}$
Vậy x = $\dfrac{-1}{8}$