Question

Tìm x biết

b) ( 2x + 3)^2 – (2x + 1)(2x – 1) = 22

c) ( 4x + 3)( 4x – 3) – (4x -5)^2 = 16 d) x^3 – 9x^2+ 27x – 27 = - 8

e) (x + 1)^3 – x2 (x + 3) = 2 ; f) (x – 2)^3 – x( x – 1)(x + 1) + 6x2 =5

Answer 1

b) ( 2x + 3)² – (2x + 1)(2x – 1) = 22

<=> 4x² + 12x + 9 - 4x² + 1 = 22

<=> 12x + 10 = 22

<=> 12x = 12

<=> x = 1

Vậy x = 1

c) (4x + 3)(4x – 3) – (4x - 5)² = 16

<=> 16x² - 9 - 16x² + 40x - 25 = 16

<=> 40x - 34 = 16

<=> 40x = 50

<=> \(\frac{5}{4}\)

Vậy \(x=\frac{5}{4}\)

e) (x + 1)³ – x²(x + 3) = 2

<=> x³ + 3x² + 3x + 1 - x³ - 3x² = 2

<=> 3x + 1 = 2

<=> 3x = 2

<=> x = \(\frac{2}{3}\)

Vậy \(x=\frac{2}{3}\)

f) (x – 2)³ – x(x – 1)(x + 1) + 6x² = 5

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

<=> 13x - 8 = 5

<=> 13x = 13

<=> x = 1

Vậy x = 1