KVPY Sample Paper KVPY Stream-SX Model Paper-19

  • question_answer
    Solution of the differential equation \[\frac{dx}{dx}=\frac{y\,(x-y\ell ny)}{x\,(x\ell nx-y)}\]

    A) \[\frac{x\ell nx+y\ell ny}{xy}=C\]

    B) \[\frac{x\ell nx-y\ell ny}{xy}=C\]

    C) \[\frac{\ell nx}{x}+\frac{\ell ny}{y}\ne C\]

    D) \[\frac{\ell nx}{x}-\frac{\ell ny}{y}=C\]

    Correct Answer: A

    Solution :

    Given, \[\frac{dy}{dx}=\frac{y(x-y\ell ny)}{x(x\ell nx-y)}\]\[\Rightarrow {{x}^{2}}\ell n\,\,x\,\,dy-xy\,\,dy=xy\,\,dx-{{y}^{2}}\ell n\,\,y\,\,dx\]
    \[\frac{\ell nx}{y}dy-\frac{1}{xy}dy=\frac{1}{xy}dx-\frac{\ell ny}{{{x}^{2}}}dx\](on dividing by \[{{x}^{2}}{{y}^{2}}\])
    \[\Rightarrow \frac{1}{xy}dx-\frac{\ell n\,x}{{{y}^{2}}}dy+\frac{1}{xy}dy-\frac{\ell n\,y}{{{x}^{2}}}dx=0\]\[\Rightarrow d\left( \frac{\ell nx}{y} \right)+d\left( \frac{\ell ny}{x} \right)=C\]
    On integrating both sides, we get \[x\,\ell n\,x+y\,\ell n\,y=Cxy\]


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