[tex]\it \dfrac{1}{x_1}+ \dfrac{1}{x_2}+ \dfrac{1}{x_3}= \dfrac{1}{2}\ \Rightarrow \dfrac{x_2x_3+x_1x_3+x_1x_2}{x_1x_2x_3}= \dfrac{1}{2}\ \stackrel{formulele\ Vi\grave{e}te}{\Longrightarrow}\ \dfrac{-2}{-a}= \dfrac{1}{2}\Rightarrow \\ \\ \\ \Rightarrow a=4[/tex]
