Toán Lớp 8: tim x biet 1,(x+4) ²-(x+1).(x-1)=16 2,(2x-1) ²+(x+3) ²-5.(x+7).(x-7)=0 3,6x.(x-4)+2x.(2-3x)=-25 4,5x ².(3x-2)-3x ².(5x+2)+2x.(3+8x)=21

Question

Toán Lớp 8:  tim x biet 1,(x+4) ²-(x+1).(x-1)=16 2,(2x-1) ²+(x+3) ²-5.(x+7).(x-7)=0 3,6x.(x-4)+2x.(2-3x)=-25 4,5x ².(3x-2)-3x ².(5x+2)+2x.(3+8x)=21

maitrucloan · Answer

1,(x+4)^2 -(x+1).(x-1)=16 => x^2 + 8x + 16 - x^2 + 1 = 16 => ( x^2 - x^2 ) + 8x + 16 - 16 + 1 = 0 => 8x + 1 = 0 => 8x = -1 => x = -1/8 2,(2x-1)^2 +(x+3)^2 -5.(x+7).(x-7)=0 <=>4x^2-4x+1+x^2+6x+9-5 (x^2 - 49 ) = 0 <=> 5x^2 + 2x + 10 - 5x^2 + 245 = 0 <=> 2x + 255 = 0 <=> x = 127,5 3,6x.(x-4)+2x.(2-3x)=-25 => 6x^2 - 24x + 4x - 6x^2 = -25 => ( 6x^2 - 6x^2 ) + ( 24 x + 4x ) = -25 => -20x = -25x => x = ( -25 ) : ( -20) => x = 5/4 4,5x^2 .(3x-2)-3x^2 .(5x+2)+2x.(3+8x)=21 => 15x^3 - 10x^2 - 15x^3 - 6x^2 + 6x + 16x^2 = 21 => ( 15x^3 - 15x^3 ) + ( 16x^2 - 6x^2 - 10x^2 ) + 6x =21 => 6x = 21 => x = 7/2

tuyetnhungthu · Answer

1, (x+4)^2-(x+1)(x-1)=16   &hArr; x^2 + 8x + 16 - x^2 + 1 = 16   &hArr; (x^2-x^2) + 8x = 16 - 16 - 1   &hArr; 8x = -1   &hArr; x = -1/8   Vậy S = {-1/8}   ...............................................................   2, (2x-1)^2 + (x+3)^2 - 5(x+7)(x-7) = 0   &hArr; 4x^2 - 4x + 1 + x^2 + 6x + 9 - 5x^2 + 245 = 0   &hArr; (5x^2-4x^2-x^2) + (6x-4x) = 0 - 245 - 9 - 1   &hArr; 2x = -255   &hArr; x = -255/2   Vậy S = {-255/2}   ....................................................................   3, 6x(x-4)+2x(2-3x)=-25   &hArr; 6x^2 - 24x + 4x - 6x^2 = -25   &hArr; (6x^2-6x^2) + (4x-24x) = -25   &hArr; -20x = -25   &hArr; x= 5/4   Vậy S = {5/4}   ..........................................................................   4, 5x^2(3x-2)-3x^2(5x+2)+2x(3+8x)=21   &hArr; 15x^3 - 10x^2 - 15x^3 - 6x^2 + 6x + 16x^2  = 21   &hArr; (15x^3-15x^3)+(16x^2-6x^2-10x^2) + 6x = 21   &hArr; 6x = 21   &hArr; x  = 7/2   Vậy S = {7/2}