Question

Tìm x biết a) (x - 3) ( x² + 3x +9 ) +  x ( x + 2 ) ( 2 - x) = 1

    b) (x + 1 ) ³ - ( x - 1 )³ - 6 ( x - 1 ) ² = -10

Answer 1

a) (x - 3)(x² + 3x + 9) + x(x + 2)(2  - x) = 1

=> x³ - 27 + x(4 - x²) = 1

=> x³ - 27 + 4x - x³ = 1

=> 4x - 27 = 1

=> 4x = 1 + 27

=> 4x = 28

=> x = 28 : 4 = 7

b) (x + 1)³ - (x - 1)³ - 6(x - 1)² = -10

=>(x + 1 - x + 1)[(x + 1)² + (x + 1)(x - 1) + (x - 1)²] - 6(x² - 2x + 1) = -10

=> 2(x² + 2x + 1 + x² - 1 + x² - 2x + 1) - 6x² + 12x - 6 = -10

=> 2(3x² + 1) - 6x² + 12x - 6 = -10

=> 6x²  + 2 - 6x² + 12x - 6 = -10

=> 12x - 4 = -10

=> 12x = -10 + 4

=> 12x = -6

=> x = -6 : 12 = -1/2