Soient $ A, B \in \mathcal{M}_n(\mathbb{R}) $.
  1. Montrer que: \[ \begin{vmatrix} A & B \\ B & A \end{vmatrix} = \det(A+B)\det(A-B) .\]
  2. Justifier que: \[ \begin{vmatrix} A & -B \\ B & A \end{vmatrix} \geqslant 0.\]