Tìm x: a) 4x(3x – 7) – 6(2x2 – 5x + 1) = 12 b) (5x + 3)(4x – 1) + (10x – 7)(–2x + 3) = 27 c) (8x – 5)(3x + 2) – (12x + 7)(2x – 1) = 17

Câu 16: Tìm x:

a) 4x(3x – 7) – 6(2x² – 5x + 1) = 12

b) (5x + 3)(4x – 1) + (10x – 7)(–2x + 3) = 27

c) (8x – 5)(3x + 2) – (12x + 7)(2x – 1) = 17

d) (5x + 9)(6x – 1) – (2x – 3)(15x + 1) = – 190.

Trả lời

a) Ta có 4x(3x – 7) – 6(2x² – 5x + 1) = 12

⇔ 12x² – 28x – 12x² + 30x – 6 = 12

⇔ 2x = 12 + 6

⇔ 2x = 18

⇔ x = 9

Vậy x = 9.

b) Ta có (5x + 3)(4x – 1) + (10x – 7)( – 2x + 3) = 27

⇔ 20x² – 5x + 12x – 3  – 20x² + 30x + 14x – 21 = 27

⇔ 51x = 27 + 21 + 3

⇔ 51x = 51

⇔ x = 1

Vậy x = 1.

c) Ta có (8x – 5)(3x + 2) – (12x + 7)(2x – 1) = 17

⇔ 24x² + 16x – 15x – 10  – 24x² + 12x – 14x + 7 = 17

⇔ – x = 17 – 7 + 10

⇔ x = – 20

Vậy x = – 10.

d) Ta có (5x + 9)(6x – 1) – (2x – 3)(15x + 1) = – 190

⇔ 30x² + 54x – 5x – 9  – 30x² + 45x – 2x + 3 = – 190

⇔ 92x = – 190 – 3 + 9

⇔ 92x = – 184

⇔ x = – 2

Vậy x = – 2.