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

  • question_answer
    If the roots of the equations \[p{{x}^{2}}+2qx+r=0\]and \[q{{x}^{2}}-2\sqrt{pr}x+q=0\] be real, then

    A) \[p=q\]             

    B) \[{{q}^{2}}=pr\]

    C) \[{{p}^{2}}=qr\]         

    D) \[{{r}^{2}}=pr\]

    Correct Answer: B

    Solution :

    Consider both equations
    \[p{{x}^{2}}+2qx+r=0\]                                 ... (i)
    and \[q{{x}^{2}}-2\sqrt{pr}\,.\,x+q=0\]                        ... (ii)
    Since, both the equations are quadratic and have real roots, therefore from equation (1), we have
    \[\therefore \,\,\,\,\,4{{q}^{2}}-4pr\ge 0\] (using discriminant)
    \[\Rightarrow \,\, {{q}^{2}}\ge pr\]                                             ... (iii)
    and from second equation \[4pr - 4{{q}^{2}} \ge  0\]
    \[\Rightarrow \,\,\, pr\ge {{q}^{2}}\]                                           ... (iv)
    From eqs. (iii) and (iv) we get \[{{\operatorname{q}}^{2}}= pr.\]


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