how to get the result of matrices

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Find a 2 × 2 matrix that rotates a plane by +45° (+45° means 45° counterclockwise).

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Hint: it is easy to find a square root of a diagonal matrix, $D$, with nonnegative entries $d_i$ on the diagonal: it is simply another diagonal matrix, $D^{1/2}$, where each entry is $\pm \sqrt{d_i}$.

A similar trick works to find $X$ so $X^2=A$ when $A$ is diagonalizable. If $A=PDP^{-1}$, then $X=PD^{1/2}P^{-1}$ is a solution.

In your case, your matrix $A$ is diagonalizable.