KVPY Sample Paper KVPY Stream-SX Model Paper-13

  • question_answer
    If \[f\,(x)={{x}^{2}}+\alpha {{x}^{2}}+\beta x+\gamma ,\] where \[\alpha ,\beta ,\gamma ,\] are rational numbers and two roots of \[f(x)=0\] are eccentricities of a parabola and a rectangular hyperbola, then \[\alpha +\beta +\gamma \] is equal to

    A) \[-\,1\]

    B) \[0\]

    C) \[1\]

    D) \[2\]

    Correct Answer: A

    Solution :

    We have, \[f(x)={{x}^{2}}+\alpha {{x}^{2}}+\beta x+\gamma \]
    Roots of \[f(x)\]are eccentricity of parabola and rectangular hyperbola.
    Eccentricity of parabola = 1
    Eccentricity of rectangular hyperbola \[=\sqrt{2}\]
    \[\therefore \]\[{{x}^{3}}+\alpha {{x}^{2}}+\beta x+\gamma =(x-1)(x-\sqrt{2})(x+\sqrt{2})\]                                 
    \[\Rightarrow \]\[{{x}^{3}}+\alpha {{x}^{2}}+\beta x+\gamma \] \[=(x-1)({{x}^{2}}-2)={{x}^{3}}-{{x}^{2}}-2x+2\]
    \[\therefore \]      \[\alpha =-1,\]\[\beta =-2,\]\[\gamma =2\]
    \[\therefore \]      \[\alpha +\beta +\gamma =-1-2+2=-1\]


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