`\vec{u}=(2x-1;3)`

`\vec{v}=(1;x+2)`

Để `\vec{u}` cùng phương `\vec{v}`:

`<=>\frac{2x-1}{1}=3/(x+2)`

`<=>(2x-1)(x+2)=3`

`<=>2x^2+3x-5=0`

`<=>(2x+5)(x-1)=0`

`<=>`\(\left[ \begin{array}{l}x=1\\x=-\dfrac{5}{2}\end{array} \right.\) 

`=>x_1.x_2=1. (-5/2)=-5/2`