JEE Main & Advanced Mathematics Pair of Straight Lines Question Bank Equation of lines joining the origin to the point of intersection of a curve and a line and Distance between the pair of lines

  • question_answer
    The pair of straight lines joining the origin to the points of intersection of the line \[y=2\sqrt{2}x+c\]and the circle \[{{x}^{2}}+{{y}^{2}}=2\]are at right angles, if                                     [MP PET 1996]

    A)            \[{{c}^{2}}-4=0\]                 

    B)            \[{{c}^{2}}-8=0\]

    C)            \[{{c}^{2}}-9=0\]                 

    D)            \[{{c}^{2}}-10=0\]

    Correct Answer: C

    Solution :

               Homogenising the curve, we get            \[{{x}^{2}}+{{y}^{2}}-2{{\left( \frac{y-2\sqrt{2}x}{c} \right)}^{2}}=0\]                            ?..(i)            But lines represented by (i) are perpendicular. Hence coefficient of \[{{x}^{2}}+\]coefficient of \[{{y}^{2}}=0\]            \[\Rightarrow 1-\frac{16}{{{c}^{2}}}+1-\frac{2}{{{c}^{2}}}=0\Rightarrow \frac{18}{{{c}^{2}}}=2\Rightarrow {{c}^{2}}-9=0\].


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