( x - 1 )² - 1 + x² = ( 1 - x )( x + 3 )
⇔ x² - 2x + 1 - 1 + x² = x + 3 - x² - 3x
⇔ 3x² + x - 3 = 0
⇔ x² + $\frac{x}{3}$ - 1 = 0
⇔ x² + 2 . x . $\frac{1}{6}$ + $\frac{1}{36}$ - 1 - $\frac{1}{36}$ = 0
⇔ ( x + $\frac{1}{6}$ )² = $\frac{37}{36}$
⇔ \(\left[ \begin{array}{l}x+\frac{1}{6}=\frac{\sqrt[]{37}}{6}\\x+\frac{1}{6}=-\frac{\sqrt[]{37}}{6}\end{array} \right.\)
⇔ \(\left[ \begin{array}{l}x=\frac{-1+\sqrt[]{37}}{6}\\x=\frac{-1-\sqrt[]{37}}{6}\end{array} \right.\)
