JEE Main & Advanced Mathematics Circle and System of Circles Question Bank Equations of circle, Geometrical problems regarding circle

  • question_answer
    Four distinct points \[(2k,\,3k),(1,0)(0,1)\]and \[(0,0)\] lie on a circle for                                        [DCE 2005]

    A)            \[\,k\in I\]                                   

    B)            \[k<0\]

    C)            \[0<k<1\]                                

    D)            For two values of k

    Correct Answer: D

    Solution :

               General equation of circle is,                    \[{{x}^{2}}+{{y}^{2}}+2gx+2fy+c=0\]                    It passes through (0,0), (1, 0) and (0, 1); \ \[c=0\]                    Now \[2g+1=0\Rightarrow g=-\frac{1}{2}\] and \[2f+1=0\Rightarrow f=\frac{-1}{2}\]            Hence equation of circle is            \[{{x}^{2}}+{{y}^{2}}-x-y=0\]            Point \[(2k,\,\,3k)\]lies on the circle            \\[4{{k}^{2}}+9{{k}^{2}}-5k=0\]            Þ \[13{{k}^{2}}-5k=0\]                    Þ \[k=0\] or \[k=\frac{5}{13}\].


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