$
\left[
\begin{array}{c}
\cos 90 \quad \sin 90 \\
-\sin 90 \quad \cos 90
\end{array}
\right]
\left[
\begin{array}{c}
a_1 \\
a_2
\end{array}
\right]
=
$

$
\left[
\begin{array}{c}
a_1 \\
a_2
\end{array}
\right]
$

(And yes, this was just to experiment with CSS rotations!)
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