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

  • question_answer
     The centre of the circle \[x=2+3\cos \theta \], \[y=3\sin \theta -1\] is [Karnataka CET 2005]

    A)             (3, 3)                                      

    B)          \[(2,\,\,-1)\]

    C)             \[(-2,\,\,1)\]                             

    D)             \[(-1,\,\,2)\]

    Correct Answer: B

    Solution :

     \[x=2+3\cos \theta ,y=3\sin \theta -1\]                    \[{{x}^{2}}+{{y}^{2}}=4+9{{\cos }^{2}}\theta +12\cos \theta +9{{\sin }^{2}}\theta +1-6\sin \theta \]                    \[=14+12\cos \theta -6\sin \theta \]                    =\[4(2+3\cos \theta )-2(3\sin \theta -1)+4\]                    Þ \[{{x}^{2}}+{{y}^{2}}=4x-2y+4\]                    Þ \[({{x}^{2}}-4x+4)+({{y}^{2}}+2y+1)=9\]                     Þ \[{{(x-2)}^{2}}+{{(y+1)}^{2}}=9\], \ centre is \[(2,-1)\].


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