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

  • question_answer
    If the sum of the roots of the equation \[{{x}^{2}}+px+q=0\] is three times their difference, then which one of the following is true     [Dhanbad Engg. 1968]

    A) \[9{{p}^{2}}=2q\]

    B) \[2{{q}^{2}}=9p\]

    C) \[2{{p}^{2}}=9q\]

    D) \[9{{q}^{2}}=2p\]

    Correct Answer: C

    Solution :

    Let \[\alpha ,\beta \] are roots of \[{{x}^{2}}+px+q=0\] So \[\alpha +\beta =-p\]and \[\alpha \beta =q\] Given that \[(\alpha +\beta )=3(\alpha -\beta )=-p\]Þ\[\alpha -\beta =\frac{-p}{3}\] Now \[{{(\alpha -\beta )}^{2}}={{(\alpha +\beta )}^{2}}-4\alpha \beta \] Þ \[\frac{{{p}^{2}}}{9}={{p}^{2}}-4q\]or \[2{{p}^{2}}=9q\].


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