JEE Main & Advanced Mathematics Complex Numbers and Quadratic Equations Question Bank Relation between roots and coefficients

  • question_answer
    If \[\alpha ,\beta \] are the roots of the equation \[{{x}^{2}}-(1+{{n}^{2}})x+\frac{1}{2}(1+{{n}^{2}}+{{n}^{4}})=0\]then the value of \[{{\alpha }^{2}}+{{\beta }^{2}}\] is [RPET  1996]

    A) \[2n\]

    B) \[{{n}^{3}}\]

    C) \[{{n}^{2}}\]

    D) \[2{{n}^{2}}\]

    Correct Answer: C

    Solution :

    \[\alpha +\beta =1+{{n}^{2}}\]; \[\alpha \beta =\frac{1}{2}(1+{{n}^{2}}+{{n}^{4}})\] \ \[{{\alpha }^{2}}+{{\beta }^{2}}={{(\alpha +\beta )}^{2}}-2\alpha \beta \] \[={{(1+{{n}^{2}})}^{2}}-2.\frac{1}{2}(1+{{n}^{2}}+{{n}^{4}})\] \[=1+{{n}^{4}}+2{{n}^{2}}-1-{{n}^{2}}-{{n}^{4}}\]Þ \[{{\alpha }^{2}}+{{\beta }^{2}}={{n}^{2}}\].


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