Condiția de coliniaritate este :
[tex]\it \begin{vmatrix} -1&1&1\\ \\ 1&a&1\\ \\ 4&2a+1&1\end{vmatrix}=0[/tex]
[tex]\it \ell_2-\ell_1;\ \ \ell_3-\ell_1;\ iar\ ecua\c{\it t}ia\ devine:[/tex]
[tex]\it \begin{vmatrix} -1&1&\underline{\underline{1}}\\ \\ 2&a-1&0\\ \\ 5&2a&0\end{vmatrix}=0 \Rightarrow \begin{vmatrix} 2&a-1\\ \\ 5&2a\end{vmatrix}=0 \Rightarrow 4a-5a+5=0 \Rightarrow a=5[/tex]
