$\text{ $x^{2}$ + 4x - 1 = 0 ( a = 1 , b = 4 , c = -1 ) }$
$\text{ Δ = $b^{2}$ - 4ac = $4^{2}$ - 4 . 1 . ( - 1 ) = 20}$
$\text{ Δ > 0 => Phương trình có hai $n_o$ phân biệt : }$
$x_1$ $\text{ = $\dfrac{-b + \sqrt{ Δ}}{2a}$ = $\dfrac{-4+ \sqrt{20}}{2.1}$ = -2 + $\sqrt{5}$ }$
$x_2$ $\text{ = $\dfrac{-b - \sqrt{Δ}}{2a}$ = $\dfrac{-4 - \sqrt{ 20 }}{2.1}$ = -2 - $\sqrt{5}$ }$
`=>` |$x_1$ - $x_2$ | $\text{ = }$ $\text{ | ( -2 + $\sqrt{5}$ ) - ( -2 - $\sqrt{5}$ ) |}$
$\text{ = | -2 + $\sqrt{5}$ + 2 + $\sqrt{5}$ | }$
$\text{ = | 2$\sqrt{5}$ | }$
$\text{ = 2$\sqrt{5}$ }$
