JEE Main & Advanced Mathematics Circle and System of Circles Question Bank Tangent and normal to a circle

  • question_answer
    Line \[y=x+a\sqrt{2}\] is a tangent to the circle \[{{x}^{2}}+{{y}^{2}}={{a}^{2}}\]at    [RPET 1991; MP PET 1999]

    A)            \[\left( \frac{a}{\sqrt{2}},\frac{a}{\sqrt{2}} \right)\]               

    B)            \[\left( -\frac{a}{\sqrt{2}},-\frac{a}{\sqrt{2}} \right)\]

    C)            \[\left( \frac{a}{\sqrt{2}},-\frac{a}{\sqrt{2}} \right)\]              

    D)            \[\left( -\frac{a}{\sqrt{2}},\frac{a}{\sqrt{2}} \right)\]

    Correct Answer: D

    Solution :

               Suppose that the point be (h, k). Tangent at      (h, k) is \[hx+ky={{a}^{2}}\equiv x-y=-\sqrt{2}a\]                    or \[\frac{h}{1}=\frac{k}{-1}=\frac{{{a}^{2}}}{-\sqrt{2}a}\] or \[h=-\frac{a}{\sqrt{2}},\ k=\frac{a}{\sqrt{2}}\]                    Therefore, point of contact is \[\left( -\frac{a}{\sqrt{2}},\ \frac{a}{\sqrt{2}} \right)\].


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