Đáp án:

Giải thích các bước giải:

$19.2x(x+3)-3x^2(x+2)+x(3x^2+4x-6)$

$=2x^2+6x-3x^3-6x^2+3x^3+4x^2-6x$

$=0$

$20.3x(2x^2-x)-2x^2(3x+1)+5(x^2-1)$

$=6x^3-3x^2-6x^3-2x^2+5x^2-5$

$=-5$

$21.4(x-6)-x^2(3x+2)+x(5x-4)+3x^2(x-1)$

$=4x-24-3x^3-2x^2+5x^2-4x+3x^3-3x^2$

$=-24$

$22.xy(3x^2-6xy)-3(x^3y-2x^2y^2-1)$

$=3x^3y-6x^2y^2-3x^3y+6x^2y^2+3$

$=3$

$23.3x^2-3x(x-2)=36$

$⇔3x^2-3x^2+6x=36$

$⇔6x=36$

$⇔x=6$

$24.6x(x-4)+2x(2-3x)=-25$

$⇔6x^2-24x+4x-6x^2=-25$

$⇔-20x=-25$

$⇔x=\dfrac{5}{4}$

$25.5x^2(3x-2)-3x^2(5x+2)+2x(3+8x)=21$

$⇔15x^3-10x^2-15x^3-6x^2+6x+16x^2=21$

$⇔6x=21$

$⇔x=\dfrac{7}{2}$

$26.5x(4x^2-2x+1)-2x(10x^2-5x-2)=-36$

$⇔20x^3-10x^2+5x-20x^3+10x^2+4x=-36$

$⇔9x=-36$

$⇔x=-4$

Đáp án:

Giải thích các bước giải:

\(19)\ 2x(x+3)-3x^2(x+2)+x(3x^2+4x-6)\\ =2x^2+6x-3x^3-6x^2+3x^3+4x^2-6x\\ =(2x^2-6x^2+4x^2)+(6x-6x)+(-3x^3+3x^3)\\ =0\\ 20)\ 3x(2x^2-x)-2x^2(3x+1)+5(x^2-1)\\ =6x^3-3x^2-6x^3-2x^2+5x^2-5\\ =(6x^3-6x^3)+(-3x^2-2x^2+5x^2)-5\\ =-5\\ 21)\ 4(x-6)-x^2(3x+2)+x(5x-4)+3x^2(x-1)\\ =4x-24-3x^3-2x^2+5x^2-4x+3x^3-3x^2\\ =(4x-4x)+(-3x^3+3x^3)+(-2x^2+5x^2-3x^2)-24\\ =-24\\ 22)\ xy(3x^2-6xy)-3(x^3y-2x^2y^2-1)\\ =3x^3y-6x^2y^2-3x^3y+6x^2y^2+3\\ =(3x^3y-3x^3y)+(-6x^2y^2+6x^2y^2)+3\\ =3\\ 23)\ 3x^2-3x(x-2)=36\\ ⇔3x^2-3x^2+6x=36\\ ⇔(3x^2-3x^2)+6x=36\\ ⇔6x=36\\ ⇔x=\dfrac{36}{6}=6\\ \text{Vậy x = 6}\\ 24)\ 6x(x-4)+2x(2-3x)=-25\\ ⇔6x^2-24x+4x-6x^2=-25\\ ⇔(6x^2-6x^2)+(-24x+4x)=-25\\ ⇔-20x=-25\\ ⇔x=\dfrac{-25}{-20}=\dfrac{5}{4}\\ \text{Vậy x = $\dfrac{5}{4}$}\\ 25)\ 5x^2(3x-2)-3x^2(5x+2)+2x(3+8x)=21\\ ⇔15x^3-10x^2-15x^3-6x^2+6x+16x^2=21\\ ⇔(15x^3-15x^3)+(-10x^2-6x^2+16x^2)+6x=21\\ ⇔6x=21\\ ⇔x=\dfrac{21}{6}=\dfrac{7}{2}\\ \text{Vậy x = $\dfrac72$}\\ 26)\ 5x(4x^2-2x+1)-2x(10x^2-5x-2)=-36\\ ⇔20x^3-10x^2+5x-20x^3+10x^2+4x=-36\\ ⇔(20x^3-20x^3)+(-10x^2+10x^2)+(5x+4x)=-36\\ ⇔9x=-36\\ ⇔x=\dfrac{-36}{9}=-4\\ \text{Vậy x = (- 4)}\)

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