differentiation

Question: If $u=\log ({{x}^{2}}+{{y}^{2}}),$ then $\frac{{{\partial }^{2}}u}{\partial {{x}^{2}}}+\frac{{{\partial }^{2}}u}{\partial {{y}^{2}}}=$

1) $\frac{1}{{{x}^{2}}+{{y}^{2}}}$

2) $0$

3) $\frac{{{x}^{2}}-{{y}^{2}}}{{{({{x}^{2}}+{{y}^{2}})}^{2}}}$

4) $\frac{{{y}^{2}}-{{x}^{2}}}{{{({{x}^{2}}+{{y}^{2}})}^{2}}}$

Solution:

Explanation: No Explanation

Partial differentiation

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