Đáp án:
a. 2x +2019 = x + 2020
⇔ 2x - x = 2020 - 2019
⇔ x = 1
b. 3x² + 4x + 9 = 3(x² + x – 1)
⇔ 3x² + 4x + 9 = 3x² + 3x - 3
⇔ 3x² - 3x² + 4x - 3x = - 3 - 9
⇔ x = -12
c. (x² – 9) + (x – 3) = 0
⇔ (x - 3)(x + 3) + (x + 3) = 0
⇔ (x + 3)(x + 3 + 1) = 0
⇔ (x + 3)(x + 4) = 0
1) x + 3 = 0 ⇔ x = -3
2) x + 4 = 0 ⇔ x = -4
d. x² - 1 = 1 - x
⇔ (x - 1)(x + 1) - 1 + x = 0
⇔ (x - 1)(x + 1) + (x - 1) = 0
⇔ (x - 1)(x + 1 + 1) = 0
⇔ (x - 1)(x + 2) = 0
1) x - 1 = 0 ⇔ x = 1
2) x + 2 = 0 ⇔ x = -2