KVPY Sample Paper KVPY Stream-SX Model Paper-8

  • question_answer
    Adjacent figure represents a equilateral triangle ABC of side length 2 units. Locus of vertex 'C? as the side AB slides along the coordinate axes is -

    A) \[{{x}^{2}}+{{y}^{2}}-xy+1=0\]

    B) \[{{x}^{2}}+{{y}^{2}}-xy\sqrt{3}=1\]

    C) \[{{x}^{2}}+{{y}^{2}}=1+xy\sqrt{3}\]

    D) \[{{x}^{2}}+{{y}^{2}}-xy\sqrt{3}+1=0\]

    Correct Answer: C

    Solution :

    \[A\,\,(2\cos \theta ,0)\,\,B\,\,(0,2sin\theta )\,\,C\,\,(h,k)\]
    From diagram
    \[h=2\cos \theta +2\cos \,\,(120{}^\circ -\theta )=\cos \theta +\sqrt{3}\sin \theta \]
    \[k=2\sin (120{}^\circ -\theta )=\sin \theta +\sqrt{3}\cos \theta \]
    \[{{h}^{3}}+{{k}^{2}}=4+4\sqrt{3}\,\,\sin \theta \cos \theta ;hk\]
    \[=\sqrt{3}+4\sin \theta \cos \theta \]
    now eliminating \[\theta \]
    \[{{h}^{2}}+{{k}^{3}}=\sqrt{3}\,\,hk+1\]


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