Toán Lớp 8: 1) (2x-5)(2x+5)-(2x-1)^2=3x+2 2) 9x^2-25-5(-6x+10)=0 3) (x-3)(x+2)-(x^2-6x+9) 4) x^2-12x+35=0 5) x^2-4x+12=0 ( de ba

Question

Toán Lớp 8:  1)   (2x-5)(2x+5)-(2x-1)^2=3x+2 2)   9x^2-25-5(-6x+10)=0 3)    (x-3)(x+2)-(x^2-6x+9) 4)     x^2-12x+35=0 5)     x^2-4x+12=0     ( de bai la tim x a giup em gap voi )

cobebongdem · Answer

~ gửi bạn ~   Giải đáp:       Lời giải và giải thích chi tiết:   a)  (2x - 5).(2x + 5) - (2x - 1)^2 = 3x + 2   &hArr; 4x^2 - 25 - (4x^2 - 4x + 1) = 3x + 2   &hArr; 4x^2 - 25 - 4x^2 + 4x + 1 = 3x + 2   &hArr; -24 + 4x = 3x + 2   &hArr; 4x - 3x = 2 + 24   &hArr; x = 26   ---------------------   b) 9x^2 - 25 - 5.(-6x + 10) = 0   &hArr; 9x^2 - 25 + 30x - 50 = 0   &hArr; 9x^2 + 30x - 75 = 0   &hArr; (3x)^2 + 2.3x.5 + 25 - 100 = 0   &hArr; (3x + 5)^2 - 100 = 0   &hArr; (3x + 5)^2 = 100 = (&plusmn; 10)^2   &hArr; 3x + 5 = 10 hoặc 3x + 5 = -10   &hArr; x = 5/3 hoặc x = -5   ---------------------   3) (x - 3).(x + 2) - (x^2 - 6x + 9)   = x^2 + 2x - 3x - 6 - x^2 + 6x - 9   = (x^2 - x^2) + (2x - 3x + 6x) + (-6 - 9)   = 5x - 15   -----------------------   4) x^2 - 12x + 35 = 0   &hArr; x^2 - 5x - 7x + 35 = 0   &hArr; x.(x - 5) - 7.(x - 5) = 0   &hArr; (x - 5).(x - 7) = 0   &hArr; x = 5 hoặc x = 7   ------------------------   5) x^2 - 4x + 12 = 0   &hArr; x^2 - 4x + 4 + 8 = 0   &hArr; (x - 2)^2 + 8 = 0   V&igrave; (x - 2)^2 &ge; 0 &forall; x   &rArr; (x - 2)^2 + 8 > 0 &forall; x   &rArr; Pt v&ocirc; nghiệm

truongthanhhung · Answer

a) ( 2x - 5 ) ( 2x + 5 ) - ( 2x - 1 )^2 = 3x + 2         4x^2 - 25 - 4x^2 + 4x + 1 = 3x + 2         - 24 + 4x = 3x + 2           4x - 3x = 26                    x = 26   Vậy x &isin; { 26 }   b) 9x^2 - 25 - 5 ( - 6x + 10 ) = 0        9x^2 - 25 + 30x - 50 = 0        ( 3x + 5 )^2 = 100   &rArr; 3x + 5 = &plusmn; 10   $ text{&rArr; x = 5/3 hoặc x = - 5}$   Vậy x &isin; { 5/3; - 5 }   c) ( x - 3 ) ( x + 2 ) - ( x^2 - 6x + 9 )   = x^2 + 2x - 3x - 6 - x^2 + 6x - 9   = 5x - 15   d) x^2 - 12x + 35 = 0       x^2 - 7x - 5x + 35 = 0       x ( x - 5 ) - 7 ( x - 5 ) = 0       ( x - 5 ) ( x - 7 ) = 0   $ text{&rArr; x = 5 hoặc x = 7}$   Vậy x &isin; { 5; 7 }   e) x^2 - 4x + 12 = 0       x^2 - 4x + 8 + 4 = 0      ( x - 2 )^2 + 8 = 0 ( v&ocirc; l&iacute; )   $ text{&rArr; V&ocirc; nghiệm}$