Đề: Tìm x, biết: a, x(x + 3) + x(4 - x) = 2 b, (x - 1) (x ² + x + 1) - x( x ²+ 3) = 4

Đáp án:

 $a) x.(x+3)+x.(4-x)=2$

$⇒ x^2+3x+4x-x^2=2$

$⇒ (x^2-x^2)+(3x+4x)=2$

$⇒ 7x=2$

$⇒ x= \dfrac{2}{7}$

Vậy x=$\dfrac{2}{7}$

~~~~~~~~~~~~~~

$b) (x-1).(x^2+x+1)-x.(x^2+3)=4$

$⇒ (x-1).(x^2+x.1+1^2)-x^3-3x=4$

$⇒ x^3-1-x^3-3x=4$

$⇒ -1-3x=4$

$⇒ 3x= -1-4$

$⇒3x=-5$

$ ⇒x= -\dfrac{5}{3}$ 

Vậy x= $-\dfrac{5}{3}$