Avem ecuații de gradul al II-lea
[tex]\it 2)\ 2x^2-x-1=0,\ \ a=2,\ \ b=-1,\ \ c=-1.\\ \\ \Delta=b^2-4ac=1^2-4\cdot2\cdot(-1)=1+8=9\\ \\ x_{1,2}=\dfrac{-b\pm\sqrt{\Delta}}{2a}=\dfrac{1\pm\sqrt9}{2\cdot2}=\dfrac{1\pm3}{4}\ \Rightarrow \begin{cases}\it x_1=\dfrac{1-3}{4}=\dfrac{-2}{4}=-\dfrac{1}{2}\\ \\ \it x_2=\dfrac{1+3}{4}=\dfrac{4}{4}=1\end{cases}[/tex]
