JEE Main & Advanced AIEEE Solved Paper-2007

  • question_answer
    The differential equation of all circles passing through the origin and having their centres on the X-axis is       AIEEE  Solved  Paper-2007

    A)  \[\frac{1}{2}\]  

    B)  \[1.0\text{ }g\,c{{m}^{-3}},\]    

    C)  \[90.0\text{ }g\,mo{{l}^{-1}}\]

    D) \[\text{115}\text{.0 }g\,mo{{l}^{-1}}\]

    Correct Answer: C

    Solution :

                    Use the general equation of circle \[{{x}^{2}}+{{y}^{2}}+2gx+2fy+c=0\]and eliminate g. General equation of all such circles which pass through the origin and whose centre lie on x-axis, is \[{{x}^{2}}+{{y}^{2}}+2gx=0\]                                    ...(i) On differentiating w.r.t.\[x,\]we get \[2x+2y\frac{dy}{dx}+2g=0\] \[\Rightarrow \]\[2g=-\left( 2x+2y\frac{dy}{dx} \right)\] On putting the value of 2 g in Eq. (i), we get \[{{x}^{2}}+{{y}^{2}}+\left( -2x-2y\frac{dy}{dx} \right)=0\] \[\Rightarrow \] \[{{x}^{2}}+{{y}^{2}}-2{{x}^{2}}-2xy\frac{dy}{dx}=0\] \[\Rightarrow \] \[{{y}^{2}}={{x}^{2}}+2xy\frac{dy}{dx}\] which is required equation.


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