Toán Lớp 8: 6x(2x+5)-(4x-1)(3x-2)=121 5x^2(2x-7)+5(7-2x)=0

Question

Toán Lớp 8:  6x(2x+5)-(4x-1)(3x-2)=121 5x^2(2x-7)+5(7-2x)=0

tuanh · Answer

Giải đáp+Lời giải và giải thích chi tiết:      a)  $6x.(2x+5)-(4x-1).(3x-2)=121$   $(12x^2+30x)-(12x^2-8x-3x+2)=121$   $12x^2+30x-12x^2+8x+3x-2=121$   $12x^2-12x^2+30x+8x+3x=121+2=123$   $41x=124$   $x=123:41$   $x=3$    b)  $5x^2.(2x-7)+5.(7-2x)=0$   $5x^2.(2x-7)-5.(2x-7)=0$   $(5x^2-5).(2x-7)=0$    Trường hợp 1:    $5x^2-5=0$   $5x^2=5$   $x^2=5:5=1$   $x=1$    Trường hợp 2:    $2x-7=0$   $2x=7$   $x= frac{7}{2}$     $#Lacmei2D$

maianhtuanh · Answer

#andy     [ begin{array}{l} a)   6x. left( {2x + 5}  right) -  left( {4x - 1}  right). left( {3x - 2}  right) = 121     Leftrightarrow 12{x^2} + 30x -  left( {12{x^2} - 8x - 3x + 2}  right) = 121     Leftrightarrow 12{x^2} + 30x - 12{x^2} + 8x + 3x - 2 = 121     Leftrightarrow 12{x^2} - 12{x^2} + 30x + 8x + 3x = 121 + 2     Leftrightarrow 41x = 123     Leftrightarrow x = 123:41     Leftrightarrow x = 3   b)   5{x^2}. left( {2x - 7}  right) + 5. left( {7 - 2x}  right) = 0     Leftrightarrow 5{x^2}. left( {2x - 7}  right) - 5. left( {2x - 7}  right) = 0     Leftrightarrow  left( {5{x^2} - 5}  right). left( {2x - 7}  right) = 0     Leftrightarrow  left[  begin{array}{l} 5{x^2} - 5 = 0   2x - 7 = 0  end{array}  right.     Leftrightarrow  left[  begin{array}{l} 5{x^2} = 5   2x = 7  end{array}  right.     Leftrightarrow  left[  begin{array}{l} {x^2} = 1   x =  dfrac{7}{2}  end{array}  right.     Leftrightarrow  left[  begin{array}{l} x = 1   x =  dfrac{7}{2}  end{array}  right.     Rightarrow S  in  left { {1; dfrac{7}{2}}  right }  end{array} ]