## The Rotation Matrix is an Orthogonal Transformation

## Problem 684

Let $\mathbb{R}^2$ be the vector space of size-2 column vectors. This vector space has an **inner product** defined by $ \langle \mathbf{v} , \mathbf{w} \rangle = \mathbf{v}^\trans \mathbf{w}$. A linear transformation $T : \R^2 \rightarrow \R^2$ is called an **orthogonal transformation** if for all $\mathbf{v} , \mathbf{w} \in \R^2$,

\[\langle T(\mathbf{v}) , T(\mathbf{w}) \rangle = \langle \mathbf{v} , \mathbf{w} \rangle.\]

For a fixed angle $\theta \in [0, 2 \pi )$ , define the matrix

\[ [T] = \begin{bmatrix} \cos (\theta) & – \sin ( \theta ) \\ \sin ( \theta ) & \cos ( \theta ) \end{bmatrix} \]
and the linear transformation $T : \R^2 \rightarrow \R^2$ by

\[T( \mathbf{v} ) = [T] \mathbf{v}.\]

Prove that $T$ is an orthogonal transformation.

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