JEE Main & Advanced Mathematics Circle and System of Circles Question Bank System of circles

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     The point (2, 3) is a limiting point of a coaxial system of circles of which \[{{x}^{2}}+{{y}^{2}}=9\]is a member. The co-ordinates of the other limiting point is given by          [MP PET 1993]

    A)            \[\left( \frac{18}{13},\frac{27}{13} \right)\]                            

    B)            \[\left( \frac{9}{13},\frac{6}{13} \right)\]

    C)            \[\left( \frac{18}{13},-\frac{27}{13} \right)\]                           

    D)            \[\left( -\frac{18}{13},-\frac{9}{13} \right)\]

    Correct Answer: A

    Solution :

               \[{{(x-2)}^{2}}+{{(y-3)}^{2}}=0\]                    or \[({{x}^{2}}+{{y}^{2}}-9)-4x-6y+22=0\]                    or \[({{x}^{2}}+{{y}^{2}}-9)-\lambda (2x+3y-11)=0\] represents the family of co-axial circles.                    \[C=\left( \lambda ,\ \frac{3\lambda }{2} \right)\text{  },\ r=\sqrt{{{\lambda }^{2}}+\frac{9{{\lambda }^{2}}}{4}-11\lambda +9}\]                    For limiting points \[r=0\]                    \[\Rightarrow 13{{\lambda }^{2}}-44\lambda +36=0\Rightarrow \lambda =\frac{18}{13},\ 2\]                    \[\therefore \]The limiting points are (2, 3) and \[\left[ \frac{18}{13},\ \frac{3}{2}\left( \frac{18}{13} \right) \right]\] or \[\left( \frac{18}{13},\ \frac{27}{13} \right)\].


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