Matrix A is a matrix of order 2. Identity matrix of second order is $\begin{bmatrix} 1 & 0 \\ 0 & 1 \end{bmatrix}$ For A to be an identity matrix, $\begin{bmatrix} 1 & 0 \\ 0 & 1 \end{bmatrix}$ = $\begin{bmatrix} \cos\alpha & -\sin\alpha \\ \sin\alpha & \cos\alpha \end{bmatrix}$ ⇒ cos α = 1 and sin α = 0 ⇒ cos α = cos 0° and sin σ = sin 0° ⇒ α = 0° Thus, for α = 0° , A is an identity matrix