JEE Main & Advanced Mathematics Circle and System of Circles Question Bank Tangent and normal to a circle

  • question_answer
     If a circle passes through the points of intersection of the coordinate axis with the lines \[\lambda x-y+1=0\]and \[x-2y+3=0\], then the value of \[\lambda \]is                                        [IIT 1991]

    A)            1    

    B)            2

    C)            3    

    D)            4

    Correct Answer: B

    Solution :

               Points of intersection with co-ordinate axes are  \[\left( -\frac{1}{\lambda },\ 0 \right)\text{  },\text{ }\ (0,\ 1)\] and \[(-3,\ 0),\ \left( 0,\ \frac{3}{2} \right)\].                    Equation of circle through (0, 1), (?3, 0) and \[\left( 0,\ \frac{3}{2} \right)\] is \[{{x}^{2}}+{{y}^{2}}+\frac{7x}{2}-\frac{5y}{2}+\frac{3}{2}=0\].                    It passes through \[\left( \frac{-1}{\lambda },\ 0 \right)\]                    \[\Rightarrow \frac{1}{{{\lambda }^{2}}}-\frac{7}{2\lambda }+\frac{3}{2}=0\Rightarrow 3{{\lambda }^{2}}-7\lambda +2=0\]                    \[\Rightarrow \lambda =\frac{7\pm \sqrt{49-24}}{6}=\frac{7\pm 5}{6}=2,\ \frac{1}{3}\].


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