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

  • question_answer
     If one end of the diameter is (1, 1) and other end lies on the line \[x+y=3\], then locus of centre of circle is [AMU 2005]

    A)            \[x+y=1\]                                

    B)            \[2(x-y)=5\]

    C)            \[2x+2y=5\]                            

    D)            None of these

    Correct Answer: C

    Solution :

               The other end is \[(t,\,3-t)\]            So the equation of the variable circle is            \[(x-1)(x-t)+(y-1)(y-3+t)=0\]            or \[{{x}^{2}}+{{y}^{2}}-(1+t)x-(4-t)y+3=0\]             \ The centre \[(\alpha ,\beta )\] is given by            \[\alpha =\frac{1+t}{2},\beta =\frac{4-t}{2}\]            Þ  \[2\alpha +2\beta =5\]            Hence, the locus is \[2x+2y=5\].


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