RAJASTHAN ­ PET Rajasthan PET Solved Paper-2011

  • question_answer
    If the angle between tangents drawn to \[{{x}^{2}}+{{y}^{2}}+2gx+2fy+c=0\]from (0, 0) is\[\pi /2,\]then

    A)  \[{{g}^{2}}+{{f}^{2}}=3c\]

    B)  \[{{g}^{2}}+{{f}^{2}}=2c\]

    C)  \[{{g}^{2}}+{{f}^{2}}=5c\]

    D)  \[{{g}^{2}}+{{f}^{2}}=4c\]

    Correct Answer: B

    Solution :

     Clearly, (0, 0) lies on director circle of the given circle. Now, equation of director circle is, \[{{(x+g)}^{2}}+{{(y+f)}^{2}}=2({{g}^{2}}+{{f}^{2}}-c)\] It (0, 0) lies on it, then \[{{g}^{2}}+{{f}^{2}}=2({{g}^{2}}+{{f}^{2}}-c)\] \[\Rightarrow \] \[{{g}^{2}}+{{f}^{2}}=2c\]


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