For which value of α\alphaα is the matrix A=(cosα−sinαsinαcosα)A = \begin{pmatrix} \cos \alpha & -\sin \alpha \\ \sin \alpha & \cos \alpha \end{pmatrix}A=(cosαsinα−sinαcosα) idempotent?
α=0\alpha = 0α=0
α=π/2\alpha = \pi/2α=π/2
α=π\alpha = \piα=π
None of the above