KVPY Sample Paper KVPY Stream-SX Model Paper-15

  • question_answer
    From an external point P, pair of tangent lines are drawn to the parabola \[{{y}^{2}}=4x.\] If \[{{\theta }_{1}}\] and \[{{\theta }_{2}}\] be the inclinations of these tangents with the axis of x such that \[{{\theta }_{1}}+{{\theta }_{2}}=\frac{\pi }{4}\] then the locus of P is -

    A) \[x-y+1=0\]

    B) \[x+y-1=0\]

    C) \[x-y-1=0\]       

    D) \[x+y+1=0\]

    Correct Answer: C

    Solution :

    Since, \[y=mx+\frac{1}{m}\] or \[{{m}^{2}}h-mk+1=0,\]
    We have \[{{m}_{1}}+{{m}_{2}}=\frac{k}{h},\] and \[{{m}_{1}}{{m}_{2}}=\frac{1}{h}\]
    Given \[{{\theta }_{1}},{{\theta }_{2}}=\frac{\pi }{4}\]
    \[\therefore \tan ({{\theta }_{1}}+{{\theta }_{2}})=\tan \frac{\pi }{4}\]
    \[\Rightarrow \frac{{{m}_{1}}+{{m}_{2}}}{1-{{m}_{1}}{{m}_{2}}}=1\]\[\Rightarrow \frac{k}{h}=1-\frac{1}{h}\]
    \[\Rightarrow y=x-1\]\[\Rightarrow x-y-1=0\]


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