JEE Main & Advanced Mathematics Complex Numbers and Quadratic Equations Question Bank Condition for common roots, Quadratic expressions and Position of roots

  • question_answer
    If \[{{x}^{2}}+px+1\] is a factor of the expression \[a{{x}^{3}}+bx+c\], then [IIT 1980]

    A) \[{{a}^{2}}+{{c}^{2}}=-ab\]

    B) \[{{a}^{2}}-{{c}^{2}}=-ab\]

    C) \[{{a}^{2}}-{{c}^{2}}=ab\]

    D) None of these

    Correct Answer: C

    Solution :

    Given that \[{{x}^{2}}+px+1\]is factor of\[a{{x}^{3}}+bx+c=0\], then let\[a{{x}^{3}}+bx+c\equiv ({{x}^{2}}+px+1)(ax+\lambda )\],  where \[\lambda \] is a constant. Then equating the coefficient of like powers of x on both sides, we get \[0=ap+\lambda ,\ \ b=p\lambda +a,\ c=\lambda \] \[\Rightarrow p=-\frac{\lambda }{a}=-\frac{c}{a}\] Hence \[b=\left( -\frac{c}{a} \right)\,c+a\] or \[ab={{a}^{2}}-{{c}^{2}}\].


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