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

  • question_answer
    If \[\alpha ,\beta \] be the roots of \[{{x}^{2}}-px+q=0\]and \[{\alpha }',{\beta }'\] be the roots of \[{{x}^{2}}-{p}'x+{q}'=0\], then the value of \[{{(\alpha -\alpha ')}^{2}}+{{(\beta -{\alpha }')}^{2}}+{{(a-{\beta }')}^{2}}+{{(\beta -{\beta }')}^{2}}\] is

    A) \[2\{{{p}^{2}}-2q+{{{p}'}^{2}}-2{q}'-p{p}'\}\]

    B) \[2\{{{p}^{2}}-2q+{{{p}'}^{2}}-2{q}'-q{q}'\}\]

    C) \[2\{{{p}^{2}}-2q-{{{p}'}^{2}}-2{q}'-p{p}'\}\]

    D) \[2\{{{p}^{2}}-2q-{{{p}'}^{2}}-2{q}'-q{q}'\}\]

    Correct Answer: A

    Solution :

    As given, \[\alpha +\beta =p,\]\[\alpha \beta =q,{\alpha }'+{\beta }'={p}',{\alpha }'{\beta }'=q'\] Now, \[{{(\alpha -\alpha ')}^{2}}+{{(\beta -\alpha ')}^{2}}+{{(\alpha -{\beta }')}^{2}}+{{(\beta -{\beta }')}^{2}}\] \[=2({{\alpha }^{2}}+{{\beta }^{2}})+2(\alpha {{'}^{2}}+\beta {{'}^{2}})-2\alpha '(\alpha +\beta )-2\beta '(a+\beta )\]       \[=2\left\{ {{(\alpha +\beta )}^{2}}-2\alpha \beta +{{({\alpha }'+{\beta }')}^{2}}-2\alpha '\beta '-(\alpha +\beta )({a}'+\beta ') \right\}\] \[=2\left\{ {{p}^{2}}-2q+{{{{p}'}}^{2}}-2{q}'-p{p}' \right\}\].


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