Let $\alpha$ be a non-zero element of $E$. Show, for every vector $x$ of $E$, the identity: $$\tau _ { \alpha } ( x ) = x - 2 \frac { \langle \alpha , x \rangle } { \langle \alpha , \alpha \rangle } \alpha$$
Let $\alpha$ be a non-zero element of $E$. Show, for every vector $x$ of $E$, the identity:
$$\tau _ { \alpha } ( x ) = x - 2 \frac { \langle \alpha , x \rangle } { \langle \alpha , \alpha \rangle } \alpha$$