Toán Lớp 8: Tìm x a,x^3-16x=0 b,4x^2+4x+1=0 c, (x+8)^2=121 d, (10x+9)x-(5x-1)(2x+3)=8

Question

Toán Lớp 8:  Tìm x a,x^3-16x=0 b,4x^2+4x+1=0 c, (x+8)^2=121 d, (10x+9)x-(5x-1)(2x+3)=8

dananhnhu2k4 · Answer

$@Shun$~   T&igrave;m x:   a) x^3 - 16x = 0   &hArr; x(x^2 - 16) = 0   &hArr; x(x^2 - 4^2) = 0   &hArr; x(x-4)(x+4) = 0   &hArr; $ left[ begin{matrix} x=0   x - 4 =0   x + 4 = 0 end{matrix} right.$   &hArr; $ left[ begin{matrix} x=0   x  =4   x =-4 end{matrix} right.$   Vậy x&isin;{0; &plusmn;4}   b) 4x^2 + 4x +1 = 0   &hArr;4x^2 + 2x +2x +1 = 0   &hArr; 2x(2x +1) + (2x+1) = 0   &hArr; (2x +1)(2x+1) = 0   &hArr;(2x+1)^2 = 0   &hArr;2x+1 = 0   &hArr;2x = -1   &hArr; x = - 1/2   Vậy x = - 1/2   c) (x+8)^2 = 121   &hArr; x+8 = 11   &hArr; x = 3   Vậy x = 3   d) (10x + 9)x - (5x-1)(2x+3) = 8   &hArr; 10x^2 + 9x - (5x -1)(2x+3) = 8    &hArr; 10x^2 + 9x - 5x(2x+3) - 1(2x +3) = 8   &hArr; 10x^2 + 9x - (10x^2 + 15x -2x -3) = 8   &hArr; 10x^2 + 9x - (10x^2 +13x - 3) = 8   &hArr; 10x^2 + 9x - 10x^2 -13x +3 = 8   &hArr; -4x + 3 = 8   &hArr; -4x = 5   &hArr; x = -5/4   Vậy x = -5/4

minhtuquynh · Answer

a)   x^3 - 16x = 0   => x (x^2 - 16)=0   => x (x^2-4^2)=0   => x (x-4)(x+4) = 0   =>x=0 hoặc x-4=0 hoặc x+4=0   +)x=0   +)x-4=0=>x=4   +)x+4=0=>x=-4   Vậy x in{0;-4;4}   b)   4x^2 + 4x+1=0   => (2x)^2 + 2 . 2x . 1 + 1^2 =0   => (2x+1)^2=0   =>2x+1=0   =>2x=-1   =>x=-1/2   Vậy x=-1/2   c)   (x+8)^2 = 121   => (x+8)^2 - 121=0   => (x+8)^2-  11^2=0   => (x+8-11)(x+8+11) =0   => (x-3)(x+19)=0   =>x-3=0 hoặc x+19=0   +)x-3=0=>x=3   +)x+19=0=>x=-19   Vậy x in{3;-19}   d)   (10x+9)x - (5x-1)(2x+3) = 8   => (10x . x + 9  .x) - ( 5x  . 2x + 5x . 3 - 1 . 2x - 1 . 3) = 8   => (10x^2 + 9x) - (10x^2 + 15x - 2x - 3) = 8   => 10x^2 + 9x - 10x^2 - 15x + 2x + 3 =8   => (10x^2 - 10x^2) + (9x - 15x + 2x) + 3 = 8   => -4x + 3 = 8   => -4x = 5   =>x=-5/4   Vậy x=-5/4