3.II.8D

Let $A$ be a $3 \times 3$ real matrix such that $\operatorname{det}(A)=-1, A \neq-I$, and $A^{T} A=I$, where $A^{T}$ is the transpose of $A$ and $I$ is the identity.

Show that the set $E$ of vectors $x$ for which $A x=-x$ forms a 1-dimensional subspace.

Consider the plane $\Pi$ through the origin which is orthogonal to $E$. Show that $A$ maps $\Pi$ to itself and induces a rotation of $\Pi$ by angle $\theta$, where $\cos \theta=\frac{1}{2}(\operatorname{trace}(A)+1)$. Show that $A$ is a reflection in $\Pi$ if and only if $A$ has trace 1 . [You may use the fact that $\operatorname{trace}\left(B A B^{-1}\right)=\operatorname{trace}(A)$ for any invertible matrix B.]

Prove that $\operatorname{det}(A-I)=4(\cos \theta-1)$.

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