JEE Main & Advanced Mathematics Complex Numbers and Quadratic Equations Question Bank Mock Test - Complex Numbers and Quadratic Equations

  • question_answer
    If the roots of the equation \[{{x}^{2}}+2ax+b=0\]are real and distinct and they differ by at most 2m then b lies in the interval

    A) \[({{a}^{2}},{{a}^{2}}+{{m}^{2}})\]        

    B) \[({{a}^{2}}-{{m}^{2}},{{a}^{2}})\]

    C) [\[{{a}^{2}}-{{m}^{2}},{{a}^{2}}\])         

    D) none of these

    Correct Answer: C

    Solution :

    [c] Let the roots be \[\alpha \],\[\beta \]
    \[\therefore \alpha +\beta =-2a\]and \[\alpha \beta \]=b
    Given, \[\left| \alpha -\beta  \right|\le 2m\]
    or \[{{\left| \alpha -\beta  \right|}^{2}}\le {{(2m)}^{2}}\] or\[{{(\alpha +\beta )}^{2}}-4ab\le 4{{m}^{2}}\] or \[4{{a}^{2}}-4b\le 4{{m}^{2}}\]
    \[\Rightarrow {{a}^{2}}-{{m}^{2}}\le b\]and discriminant \[D>0\]or \[4{{a}^{2}}-4b>0\]
    \[\Rightarrow {{a}^{2}}-{{m}^{2}}\le b\] and \[b<{{a}^{2}}\].
    Hence, \[b\in [{{a}^{2}}-{{m}^{2}},{{a}^{2}}).\]
     


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