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

  • question_answer
    If a, b are the roots of \[9{{x}^{2}}+6x+1=0,\] then the equation with the roots \[\frac{1}{\alpha },\,\frac{1}{\beta }\] is   [EAMCET  2000]

    A) \[2{{x}^{2}}+3x+18=0\]

    B) \[{{x}^{2}}+6x-9=0\]

    C) \[{{x}^{2}}+6x+9=0\]

    D) \[{{x}^{2}}-6x+9=0\]

    Correct Answer: C

    Solution :

    Given equation is \[9{{x}^{2}}+6x+1=0\] \[\Rightarrow \alpha +\beta =\frac{-6}{9}=\frac{-2}{3}\] and \[\alpha \beta =1/9\] \[\therefore \alpha -\beta =\sqrt{{{(\alpha +\beta )}^{2}}-4\alpha \beta }\] \[=\sqrt{\frac{4}{9}-4.\frac{1}{9}}=0\] \[\Rightarrow \alpha =\frac{-1}{3},\beta =\frac{-1}{3}\]    \ Equation \[{{x}^{2}}-(\alpha +\beta )x+\alpha \beta =0\] \[\Rightarrow {{x}^{2}}+6x+9=0\].


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