Toán Lớp 8: Câu 3: Tìm x biết a)(x + 2)(x²– 2x + 4) – x( x²– 2) = 12 b) (x

Question

Toán Lớp 8:  Câu 3: Tìm x biết                         a)(x + 2)(x²– 2x + 4) – x( x²– 2) = 12                                                  b) (x +4)² – (x+1)(x-1) = 16 c)x³–  16x = 0                                               d) x³ – 5x² +9x – 45 = 0

diemmyhoa · Answer

a)(x+2)(x&sup2;-2x+4)-x(x&sup2;-2)=12    x&sup3;-2x&sup2;+4x+2x&sup2;-4x+8-x&sup3;+2x=12    8+2x=12   2x=12-8   2x=4   x=4:2   X=2   b)

lantrungbach · Answer

a,  (x+2) (x&sup2;-2x+4) - x (x&sup2;-2) = 12    &rArr; x&sup3; - 2x&sup2; + 4x + 2x&sup2; - 4x + 8 - x&sup3; + 2x = 12   &rArr; ( x&sup3; - x&sup3; ) + ( -2x&sup2; + 2x&sup2; ) + ( 4x - 4x ) + 8 + 2x = 12   &rArr; 8 + 2x = 12   &rArr; 2x        = 12-8   &rArr; 2x         = 4   &rArr;    x        = 4 : 2   &rArr;     x        = 2    V&acirc;y x = 2    b,   (x +4)&sup2; &ndash; (x+1) (x-1) = 16     &rArr; (x +4)&sup2; - x&sup2; - 1&sup2;        = 16     &rArr; x&sup2; + 8x + 16  - x&sup2; - 1&sup2; = 16     &rArr; ( x&sup2; - x&sup2; ) + 8x + 16 - 1&sup2; =16     &rArr; 8x + 16 - 1                    = 16    &rArr; 8x + 15                          = 16   &rArr;  8x                                  = 16 - 15   &rArr; 8x                                    = 1   &rArr;   x                                     = 1 : 8   &rArr;   x                                     = $ frac{1}{8}$    Vậy x = $ frac{1}{8}$    c,    x&sup3; &ndash; 16x = 0     &rArr; x (x&sup2; - 16 ) = 0     &rArr; x = 0         x&sup2; -16 = 0     &rArr;x = 0        x = &plusmn;4      Vậy x = 0 hoặc x = &plusmn; 4      d,  x&sup3; &ndash; 5x&sup2; +9x &ndash; 45 = 0     &rArr; x&sup2; ( x - 5 ) + 9 ( x - 5 ) = 0     &rArr; ( x - 5 ) ( x&sup2; + 9 ) = 0     &rArr; x - 5 = 0         x&sup2; + 9 = 0     &rArr; x = 5            x = - ( &plusmn;3 )      Vậy x = 5 hoặc x = - ( &plusmn;3 )                                                    ~ Ch&uacute;c bạn học tốt ! ~