Toán Lớp 8: Tìm x biết a,X²-3x=0 b,X²-2x+1=0 c,4x²-4x=-1 d,36²=(5-x)² e,X(x-1)-x+1=0 g,X²+5x+6=0

Question

Toán Lớp 8:  Tìm x biết  a,X²-3x=0 b,X²-2x+1=0 c,4x²-4x=-1 d,36²=(5-x)² e,X(x-1)-x+1=0 g,X²+5x+6=0

thanhthuythu · Answer

Giải đáp:     a, x^2 - 3x = 0   &harr; x.(x - 3) = 0   &harr; $ left[ begin{matrix} x = 0   x- 3 = 0 end{matrix} right.$ &harr; $ left[ begin{matrix} x = 0   x = 3 end{matrix} right.$    Vậy x &isin; {0; 3}   --------------   b, x^2 - 2x + 1 = 0   &harr; x^2 - 2.x.1 + 1^2 = 0   &harr; (x - 1)^2 = 0   &harr; x - 1 = 0   &harr; x = 1   Vậy x = 1   ---------------   c, 4x^2 - 4x = -1   &harr; 4x^2 - 4x + 1 = 0   &harr; (2x)^2 - 2.2x.1 + 1^2 =0   &harr; (2x - 1)^2 = 0   &harr; 2x - 1 = 0   &harr; 2x = 1   &harr; x = 1/2   Vậy x = 1/2   ---------------   d, 36^2 = (5 - x)^2   &harr; 36^2 - (5 - x)^2 = 0   &harr; [36 - (5 - x)].[36 + (5 - x)] = 0   &harr; (36 - 5 + x).(36 + 5 - x) = 0   &harr; (31 + x).(41 - x) = 0   &harr; $ left[ begin{matrix} 31 + x = 0   41 - x = 0 end{matrix} right.$ &harr; $ left[ begin{matrix} x = - 31   x = 41 end{matrix} right.$   Vậy x &isin; { -31; 41}   ---------------   e, x.(x - 1) - x + 1 = 0   &harr; x.(x - 1) - (x - 1) = 0   &harr; (x - 1).(x - 1) = 0   &harr; (x - 1)^2 = 0   &harr; x - 1 = 0   &harr; x = 1   Vậy x = 1   -----------------   g, x^2 + 5x + 6 = 0   &harr; x^2 +2x + 3x + 6 = 0   &harr; x.(x + 2) + 3.(x + 2) = 0   &harr; (x + 2).(x + 3) = 0   &harr; $ left[ begin{matrix} x + 2 = 0   x + 3 = 0 end{matrix} right.$ &harr; $ left[ begin{matrix} x = - 2   x = -3 end{matrix} right.$   Vậy x &isin; {-2; -3}   #dariana

behocgioitoan · Answer

a) x^2 - 3x = 0     &hArr; x ( x - 3 ) = 0     &hArr;  ( left[  begin{array}{l}x=0  x-3=0 end{array}  right. )      &hArr;  ( left[  begin{array}{l}x=0  x=3 end{array}  right. )      Vậy x = 0 ; x = 3     b) x^2 - 2x + 1 = 0     &hArr; ( x - 1 )^2 = 0     &hArr; x - 1 = 0     &hArr; x = 1     Vậy x = 1     c) 4x^2 - 4x = - 1     &hArr; 4x^2 - 4x + 1 = 0     &hArr; ( 2x - 1 )^2 = 0     &hArr; 2x - 1 = 0     &hArr; 2x = 1     &hArr; x = 1/2     Vậy x = 1/2     d) 36^2 = ( 5 - x )^2     &hArr; ( 5 - x )^2 = 36^2     &hArr; 5 - x = &plusmn; 36     &hArr;  ( left[  begin{array}{l}5-x=36  5-x=-36 end{array}  right. )      &hArr;  ( left[  begin{array}{l}x=-31  x=41 end{array}  right. )      Vậy x = - 31 ; x = 41     e) x ( x - 1 ) - x + 1 = 0     &hArr; x ( x - 1 ) - ( x - 1 ) = 0     &hArr; ( x - 1 ) ( x - 1 ) = 0     &hArr; ( x - 1 )^2 = 0     &hArr; x - 1 = 0     &hArr; x = 1     Vậy x = 1     g) x^2 + 5x + 6 = 0     &hArr; x^2 + 3x + 2x + 6 = 0     &hArr; x ( x + 3 ) + 2 ( x + 3 ) = 0     &hArr; ( x + 2 ) ( x + 3 ) = 0     &hArr;  ( left[  begin{array}{l}x+2=0  x+3=0 end{array}  right. )      &hArr;  ( left[  begin{array}{l}x=-2  x=-3 end{array}  right. )      Vậy x = - 2 ; x = - 3