CET Karnataka Engineering CET - Karnataka Engineering Solved Paper-2009

  • question_answer
    The differential equation of the family of circles passing through the orign and having their centres on the x-axis is

    A)  \[{{y}^{2}}={{x}^{2}}+2xy\,\frac{dy}{dx}\]

    B)  \[{{y}^{2}}={{x}^{2}}-2xy\,\frac{dy}{dx}\]

    C)  \[{{x}^{2}}={{y}^{2}}+xy\,\frac{dy}{dx}\]

    D)  \[{{x}^{2}}={{y}^{2}}+3xy\,\frac{dy}{dx}\]

    Correct Answer: A

    Solution :

    The system of circles passing through origin and centre lies on x-axis is \[{{x}^{2}}+{{y}^{2}}-2hx=0\]     ?..(i)                 On differentiating w. r. t. x, we get                                 \[2x+2y\frac{dy}{dx}-2h=0\]                 \[\Rightarrow \] \[2x+2y\frac{dy}{dx}-\left( \frac{{{x}^{2}}+{{y}^{2}}}{x} \right)=0\]  [from Eq. (i)]                 \[\Rightarrow \]               \[{{x}^{2}}-{{y}^{2}}+2xy\frac{dy}{dx}=0\]


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