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

  • question_answer
    If a circle passes through the point (1, 2) and cuts the circle \[{{x}^{2}}+{{y}^{2}}=4\] orthogonally, then the equation of the locus of its centre is [MNR 1992]

    A)            \[{{x}^{2}}+{{y}^{2}}-3x-8y+1=0\]                              

    B)            \[{{x}^{2}}+{{y}^{2}}-2x-6y-7=0\]

    C)            \[2x+4y-9=0\]                        

    D)            \[2x+4y-1=0\]

    Correct Answer: C

    Solution :

               Let equation of circle be \[{{x}^{2}}+{{y}^{2}}+2gx+2fy+c=0\]                    It passes through (1, 2) i.e. \[1+4+2g+4f+c=0\] and is orthogonal w.r.t\[{{x}^{2}}+{{y}^{2}}=4\].                    Therefore, \[2g\times 0+2f\times 0=-c+4\] or \[c=4\]                    Hence, we get\[2g+4f+9=0\],                    then\[2x+4y-9=0\] is the required locus.


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