Question

Tìm x, biết :
a) 4x(x-5)-(x-1)(1x-3)=23

b) (x-5)(x-4)-(x+1)(x-2)=7

Answer 1

a) `4x(x-5)-(x-1)(x-3)=23`

`<=> 4x^2-20x-x^2+4x-3=23`

`<=>3x^2-16x-26=0`

`<=> x=(8+-\sqrt142)/3

*Nếu đề là: `4x(x-5)-(x-1)(4x-3)=23`

`<=> 4x^2-20x-4x^2+7x-3=23`

`<=>-13x=26`

`<=>x=-2`

b) `(x-5)(x-4)-(x+1)(x-2)=7`

`<=>x^2-9x+20-x^2+x+2=7`

`<=>-8x=-15`

`<=>x=15/8`

Answer 2

\(\text{a) 4x(x-5)-(x-1)(1x-3)=23}\)

\(\Leftrightarrow4x^2-20x-\left(x^2-3x-x+3\right)=23\)

\(\Leftrightarrow4x^2-20x-x^2+3x+x-3=23\)

\(\Leftrightarrow3x^2-16x-26=0\)

Giải phương trình bậc 2 một ẩn ta được :

\(x_1=\dfrac{8-\sqrt{142}}{3};x_2=\dfrac{8+\sqrt{142}}{3}\)