[tex]d_1:~~x-y=0\\ d_2:~~2x-y+1=0\\d_3:~~x+ay+1=0\\ \\ \text{Cele 3 drepte sunt concurente }\Rightarrow \left|\begin{array}{ccc}1&-1&0\\2&-1&1\\1&a&1\end{array}\right| = 0 \Rightarrow \\ \\ \Rightarrow -1+0-1-0-a+2 = 0 \Rightarrow -2-a+2 = 0 \Rightarrow \boxed{a = 0}[/tex]
