Tìm giá trị của x biết (6x + 7)^2 . 2.(3x + 4). 6.(x + 1) = 72

(6x + 7)² . 2.(3x + 4). 6.(x + 1) = 72

(6x + 7)² . (6x + 8) . (6x + 6) = 72

(6x + 7)² . ( 6x + 7 + 1). (6x + 7 – 1) = 72

(6x + 7)² . [ (6x + 7)² – 1] = 72

(6x +7)⁴ − (6x + 7)² = 72

(6x +7)⁴ − 9. (6x + 7)² + 8. (6x + 7)² − 72 = 0

(6x + 7)² . [(6x + 7)² − 9] + 8. [ (6x + 7)² − 9] = 0

[(6x + 7)² − 9] . [ (6x + 7)² + 8] = 0

(6x + 7)² − 9 = 0 vì (6x + 7)² + 8 > 0 với mọi x

(6x + 7)² = 9

6x + 7 = 3 hoặc 6x + 7 = −3

• 6x + 7 = 3

6x = 3 – 7

6x = −4

x =

• 6x + 7 = −3

6x = −3 – 7

x =

Vậy x = , x =