JEE Main & Advanced Mathematics Differentiation Question Bank Partial Differentiation

  • question_answer
    If \[{{z}^{2}}=\frac{{{x}^{1/2}}+{{y}^{1/2}}}{{{x}^{1/3}}+{{y}^{1/3}}}\] then \[x\frac{\partial z}{\partial x}+y\frac{\partial z}{\partial y}=\]   [EAMCET 1999]

    A)            \[\frac{z}{6}\]

    B)            \[\frac{z}{3}\]

    C)            \[\frac{z}{2}\]

    D)            \[\frac{z}{12}\]

    Correct Answer: D

    Solution :

                       \[{{z}^{2}}\] is homogeneous in x, y of degree \[\frac{1}{6}\]                    \[\therefore \] \[x\,\frac{\partial }{\partial x}({{z}^{2}})+y\frac{\partial }{\partial y}({{z}^{2}})=\frac{1}{6}({{z}^{2}})\]                    Þ \[2xz\frac{\partial z}{\partial x}+2yz\frac{\partial z}{\partial y}=\frac{1}{6}{{z}^{2}}\]Þ \[x\frac{\partial z}{\partial x}+y\frac{\partial z}{\partial y}=\frac{1}{12}z\].


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