Question

Va rog mult la sub 2 ex 1 b)

Answer 1

$A(a)\text{ neinversabil\u a}\Leftrightarrow det(A(a))=0\\\\det(A(a))=\left|\begin{array}{ccc}1&1&1\\1&3&a\\1&9&a^2\end{array}\right|\overset{Vandermonde}{=}(3-1)(a-1)(a-3)=2(a-1)(a-3)\\\\det(A(a))=0\Rightarrow 2(a-1)(a-3)=0\Rightarrow a\in\{1, 3\}$