L.H.S: A + A' = \(\begin{pmatrix} cosα & sinα \\ -sinα & cosα \\ \end{pmatrix}\) + \(\begin{pmatrix} cosα & -sinα \\ sinα & cosα \\ \end{pmatrix}\)
This will be equal to \(
\begin{pmatrix}
1 & 0 \\
0 & 1 \\
\end{pmatrix}\)
When 2cosa = 1
cosa = 1/2
a = π/2