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

  • question_answer
    If the roots of the equation \[A{{x}^{2}}+Bx+C=0\] are \[\alpha ,\beta \] and the roots of the equation \[{{x}^{2}}+px+q=0\] are \[{{\alpha }^{2}},\ {{\beta }^{2}}\], then value of p will be [RPET 1986]

    A) \[\frac{{{B}^{2}}-2AC}{{{A}^{2}}}\]

    B) \[\frac{2AC-{{B}^{2}}}{{{A}^{2}}}\]

    C) \[\frac{{{B}^{2}}-4AC}{{{A}^{2}}}\]

    D) None of these

    Correct Answer: B

    Solution :

    \[\alpha ,\beta \] are the roots of \[A{{x}^{2}}+Bx+C=0\]. So, \[\alpha +\beta =-\frac{B}{A}\]and \[\alpha \beta =\frac{C}{A}\] Again \[{{\alpha }^{2}},{{\beta }^{2}}\] are the roots of \[{{x}^{2}}+px+q=0\] then \[{{\alpha }^{2}}+{{\beta }^{2}}=-p\] and\[{{(\alpha \beta )}^{2}}=q\] Now \[{{\alpha }^{2}}+{{\beta }^{2}}={{(\alpha +\beta )}^{2}}-2\alpha \beta \] Þ  \[{{\alpha }^{2}}+{{\beta }^{2}}={{\left( -\frac{B}{A} \right)}^{2}}-2\frac{C}{A}\] Þ  \[-p=\frac{{{B}^{2}}-2AC}{{{A}^{2}}}\Rightarrow p=\frac{2AC-{{B}^{2}}}{{{A}^{2}}}\]


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