Problem 1250 (difficulty: 5/10)

Find the derivative of \(\displaystyle \mathrm{tr}:\R^{n\times n}\to\R\), \(\displaystyle \mathrm{tr}\begin{pmatrix} a_{11} & \dots & a_{1n} \\ \vdots & \ddots & \vdots \\ a_{n1} & \dots & a_{nn} \\ \end{pmatrix}=a_{11}+a_{22}+\ldots+a_{nn}\).

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