12. 5x²−x−1=0

\(D=(-1)^2-4\cdot5\cdot(-1)=21\).

\(x_{1,2}=\frac{1\pm\sqrt{21}}{10}\).

Ответ: \(x_1=\frac{1+\sqrt{21}}{10},\ x_2=\frac{1-\sqrt{21}}{10}\).