KVPY Sample Paper KVPY Stream-SX Model Paper-24

  • question_answer
    If \[cos\text{ }\alpha ,\] \[cos\text{ }\beta \] and \[cos\text{ }\gamma \] are the roots of the equation \[9{{x}^{3}}-9{{x}^{2}}-x+1=0,\] \[\alpha ,\beta ,\gamma ,\]\[\in [0,\pi ],\] then radius of the circle whose centre is \[\left( \sum{a},\sum{\cos \alpha } \right)\] and passing through  \[\left( 2{{\sin }^{-1}}\left( \tan \frac{\pi }{4} \right),4 \right)\] is-

    A) 3

    B) 9   

    C) 4

    D) 6

    Correct Answer: A

    Solution :

    \[9{{x}^{3}}-9{{x}^{2}}-x+1=0\] \[\Rightarrow (x-1)\,\,(9{{x}^{2}}-1)=0\] \[\Rightarrow x=1,\,\,\pm \frac{1}{3}\]
    \[\cos \alpha =1\]           \[\Rightarrow \alpha =0\]
    \[\cos \beta =1/3\]          \[\Rightarrow \beta =co{{s}^{-1}}1/3\]
    \[\cos \gamma =1/3\]      \[\Rightarrow \gamma =\pi -{{\cos }^{-1}}1/3\]
    \[\sum \alpha =\pi ,\]\[\sum \cos \alpha =1\]
    \[\therefore \] center is \[(\pi ,1)\]
    \[\therefore \] equation of the circle is \[{{(x-\pi )}^{2}}+{{(y-1)}^{2}}={{r}^{2}}\]
    Which passes through \[(2si{{n}^{-1}}\left( \tan \frac{\pi }{4} \right),4)\equiv (\pi ,4)\]
    \[\therefore \,\,\,0+9={{r}^{2}}\Rightarrow r=3\]


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