JEE Main & Advanced Mathematics Differentiation Question Bank Partial Differentiation

  • question_answer
    If \[F(u)=f(x,\,y,\,z)\] be a homogeneous function of degree \[n\] in \[x,\,y,\,z\] then \[x\frac{\partial u}{\partial x}+y\frac{\partial u}{\partial y}+z\frac{\partial u}{\partial z}=\]

    A)            \[nu\]

    B)            \[n\,F(u)\]

    C)            \[\frac{n\,F(u)}{{F}'(u)}\]

    D)            None of these

    Correct Answer: C

    Solution :

                       Since \[F(u)\] is homogeneous in \[x,y,z\] of degree n.                    \[\therefore \] \[x\frac{\partial }{\partial x}(F(u))+y.\frac{\partial }{\partial y}(F(u))+z\frac{\partial }{\partial z}(F(u))=nF(u)\]                    Þ \[x.{F}'(u)\frac{\partial u}{\partial x}+y{F}'(u)\frac{\partial u}{\partial y}+z{F}'(u)\frac{\partial u}{\partial z}=nF(u)\]                    Þ \[\frac{{{\partial }^{2}}z}{\partial {{x}^{2}}}={{a}^{2}}{{\sec }^{3}}(y-ax)+{{a}^{2}}\sec (y-ax){{\tan }^{2}}(y-ax)\].


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