RAJASTHAN ­ PET Rajasthan PET Solved Paper-2002

  • question_answer
    The point from which the length of tangents to the circles\[{{x}^{2}}+{{y}^{2}}-8x+40=0,\]\[5{{x}^{2}}+5{{y}^{2}}-25x+80=0\]and \[{{x}^{2}}+{{y}^{2}}-8x+16y+160=0\]are equal, is

    A)  \[\left( 8,\frac{15}{2} \right)\]

    B)  \[\left( -8,\frac{15}{2} \right)\]

    C)  \[\left( 8,-\frac{15}{2} \right)\]

    D)  None of these

    Correct Answer: C

    Solution :

     Equations of circles \[{{x}^{2}}+{{y}^{2}}-8x+40=0\]                 ...(i) \[5{{x}^{2}}+5{{y}^{2}}-25x+80=0\] \[\Rightarrow \]    \[{{x}^{2}}+{{y}^{2}}-5x+16=0\]            ...(ii) and   \[{{x}^{2}}+{{y}^{2}}-8x+16y+160=0\]        ...(iii) Radical axis of Eqs. (i) and (ii), \[({{x}^{2}}+{{y}^{2}}-8x+40)-({{x}^{2}}+{{y}^{2}}-5x+16)=0\] \[\Rightarrow \] \[-8x+5x+40-16=0\] \[\Rightarrow \] \[3x=24\] \[\Rightarrow \] \[x=8\] Radical axis of Eqs. (i) and (iii), \[({{x}^{2}}+{{y}^{2}}-8x+40)-({{x}^{2}}+{{y}^{2}}-8x\] \[+16y+160)=0\] \[\Rightarrow \] \[-\text{ }8x+40+8x-16y-160=0\] \[\Rightarrow \] \[-16y-120=0\] \[\Rightarrow \] \[y=-\frac{120}{16}=-\frac{15}{2}\]\[\left( 8,-\frac{15}{2} \right)\] Thus, required point is


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