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

  • question_answer
    If  \[\alpha \]and \[\beta \] be the roots of the equation \[2{{x}^{2}}+2(a+b)x+{{a}^{2}}+{{b}^{2}}=0\], then the equation whose roots are \[{{(\alpha +\beta )}^{2}}\]and \[{{(\alpha -\beta )}^{2}}\] is

    A) \[{{x}^{2}}-2abx-{{({{a}^{2}}-{{b}^{2}})}^{2}}=0\]

    B) \[{{x}^{2}}-4abx-{{({{a}^{2}}-{{b}^{2}})}^{2}}=0\]

    C) \[{{x}^{2}}-4abx+{{({{a}^{2}}-{{b}^{2}})}^{2}}=0\]

    D) None of these

    Correct Answer: B

    Solution :

    Sum of roots \[\alpha +\beta =-(a+b)\]and \[\alpha \beta =\frac{{{a}^{2}}+{{b}^{2}}}{2}\] Þ \[{{(\alpha +\beta )}^{2}}={{(a+b)}^{2}}\]and \[{{(\alpha -\beta )}^{2}}={{\alpha }^{2}}+{{\beta }^{2}}-2\alpha \beta \] = \[2ab-({{a}^{2}}+{{b}^{2}})=-{{(a-b)}^{2}}\] Now the required equation whose roots are \[{{(\alpha +\beta )}^{2}}\]and \[{{(\alpha -\beta )}^{2}}\] \[{{x}^{2}}-\{{{(\alpha +\beta )}^{2}}+{{(\alpha -\beta )}^{2}}\}\,x+{{(\alpha +\beta )}^{2}}{{(\alpha -\beta )}^{2}}=0\] \[\Rightarrow {{x}^{2}}-\{{{(a+b)}^{2}}-{{(a-b)}^{2}}\}\,x-{{(a+b)}^{2}}{{(a-b)}^{2}}=0\] \[\Rightarrow {{x}^{2}}-4abx-{{({{a}^{2}}-{{b}^{2}})}^{2}}=0\]


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