KVPY Sample Paper KVPY Stream-SX Model Paper-15

  • question_answer
    If the line \[y=\sqrt{3}x\] intersects the curve \[{{x}^{3}}+{{y}^{3}}+3xy+5{{x}^{2}}+\text{4}x+5y-1=0\]  at the points A, B, C then \[OA.\,\,OB.\,\,OC\]is (Here \['O'\] is origin)

    A) \[\frac{4}{13}\,\,(3\sqrt{3}+1)\] 

    B) \[\frac{4}{13}\,\,(3\sqrt{3}-1)\]

    C) \[\frac{1}{26}\,\,(3\sqrt{3}-1)\]   

    D) \[\frac{1}{26}\,\,(3\sqrt{3}+1)\]

    Correct Answer: B

    Solution :

    Line \[y=\sqrt{3}x\] ?.(1)
    and curve
    \[{{x}^{3}}+{{y}^{3}}+3xy+5{{x}^{2}}+3{{y}^{2}}+4x+5y-1=0\] ?. (2)
    Solving (1) & (2) then
    \[\Rightarrow {{x}^{3}}+3\sqrt{3}{{x}^{3}}+3\sqrt{3}\,{{x}^{2}}\]\[+\,\,5{{x}^{2}}+9{{x}^{2}}+4x+5\sqrt{3}\,\,x-1=0\]
    Let roots \[{{x}_{1}},{{x}_{2}},{{x}_{3}}\]
    Then \[{{x}_{1}}{{x}_{2}}{{x}_{3}}=\frac{1}{3\sqrt{3}+1}\]
    Co-ordinates of A, B, C are \[({{x}_{1}},\,\,\sqrt{3}\,\,{{x}_{1}}),\] \[({{x}_{2}},\,\,\sqrt{3}\,\,{{x}_{2}})\] and \[({{x}_{3}},\,\,\sqrt{3}\,\,{{x}_{3}})\] respestively.
    then \[OA.OB.OC=8{{x}_{1}}\,{{x}_{2}}\,{{x}_{3}}\]
    \[=\frac{8}{3\sqrt{3}+1}=\frac{8\,(3\sqrt{3}-1)}{26}=\frac{4}{13}\,\,(3\sqrt{3}-1)\]


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