Question

Tìm x, biết:

a) x3 - 9x² + 27x - 27 = -8

b) 64x³ + 48x² + 12x + 1 = 27

c) (2x - 1)³ - 4x². (2x - 3) = 5

d) (x + 4)³ - x². (x + 12) = 16

Answer 1

a) x³ - 9x² + 27x - 27 = -8

<=> x³ - 3x².3 + 3x.3² - 3³ = -8

<=> (x - 3)³ = -2³

<=> x - 3 = -2

<=> x = 1 (T/m)

Vậy x = 1.

Answer 2

b) 64x³ + 48x² + 12x + 1 = 27

<=> (4x)³ + 3.(4x)².1 + 3.4x.1² + 1³ = 27

<=> (4x + 1)³ = 3³

<=> 4x + 1 = 3

<=> 4x = 2

<=> x = \(\frac{1}{2}\)(T/m)

Vậy x = \(\frac{1}{2}\).

Answer 3

c) (2x - 1)³ - 4x².(2x - 3) = 5

<=> (8x³ - 12x² + 6x - 1) - (8x³ - 12x²) = 5

<=> 8x³ - 12x² + 6x - 1 - 8x³ + 12x² = 5

<=> 6x - 1 = 5

<=> 6x = 6

<=> x = 1 (T/m)

Vậy x = 1.

Answer 4

d) (x + 4)³ - x².(x + 12) = 16

<=> (x³ + 12x² + 48x + 64) - (x³ + 12x²) = 16

<=> x³ + 12x² + 48x + 64 - x³ - 12x² = 16

<=> 48x + 64 = 16

<=> 48x = -48

<=> x = -1 (T/m)

Vậy x = -1.

Answer 5

c) (2x - 1)3 - 4x2.(2x - 3) = 5

<=> (8x3 - 12x2 + 6x - 1) - (8x3 - 12x2) = 5

<=> 8x3 - 12x2 + 6x - 1 - 8x3 + 12x2 = 5

<=> 6x - 1 = 5

<=> 6x = 6

<=> x = 1 (T/m)

Vậy x = 1.

Answer 6

b) 64x3 + 48x2 + 12x + 1 = 27

<=> (4x)3 + 3.(4x)2.1 + 3.4x.12 + 13 = 27

<=> (4x + 1)3 = 33

<=> 4x + 1 = 3

<=> 4x = 2

<=> x = (T/m)

Vậy x = .

Answer 7

b, x=1/2 nhe