JEE Main & Advanced Mathematics Differential Equations Question Bank Formation of differential equations

  • question_answer
    The differential equation of all circles which passes through the origin and whose centre lies on y-axis, is [MNR 1986; DCE 2000]

    A)                 \[({{x}^{2}}-{{y}^{2}})\frac{dy}{dx}-2xy=0\]         

    B)                 \[({{x}^{2}}-{{y}^{2}})\frac{dy}{dx}+2xy=0\]

    C)                 \[({{x}^{2}}-{{y}^{2}})\frac{dy}{dx}-xy=0\]           

    D)                 \[({{x}^{2}}-{{y}^{2}})\frac{dy}{dx}+xy=0\]

    Correct Answer: A

    Solution :

                       The system of circles pass through origin and centre lies on y-axis is \[{{x}^{2}}+{{y}^{2}}-2ay=0\]                    Þ \[2x+2y\frac{dy}{dx}-2a\frac{dy}{dx}=0\] Þ \[2a=2y+2x\frac{dx}{dy}\]                    Therefore, the required differential equation is                                 \[{{x}^{2}}+{{y}^{2}}-2{{y}^{2}}-2xy\frac{dx}{dy}=0\]Þ\[({{x}^{2}}-{{y}^{2}})\frac{dy}{dx}-2xy=0\].


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