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

  • question_answer
    If a root of the equations \[{{x}^{2}}+px+q=0\] and \[{{x}^{2}}+\alpha x+\beta =0\] is common, then its value will be (where \[p\ne \alpha \] and \[q\ne \beta \]) [IIT 1974, 1976; RPET 1997]

    A) \[\frac{q-\beta }{\alpha -p}\]

    B) \[\frac{p\beta -\alpha q}{q-\beta }\]

    C)  \[\frac{q-\beta }{\alpha -p}\]or \[\frac{p\beta -\alpha q}{q-\beta }\]

    D) None of these

    Correct Answer: C

    Solution :

    Let the common root be y. Then \[{{y}^{2}}+py+q=0\] and \[{{y}^{2}}+\alpha \text{ }y+\beta =0\] On solving by cross multiplication, we have \[\frac{{{y}^{2}}}{p\beta -q\alpha }=\frac{y}{q-\beta }=\frac{1}{\alpha -p}\] \ \[y=\frac{q-\beta }{\alpha -p}\]and \[\frac{{{y}^{2}}}{y}=y=\frac{p\beta -q\alpha }{q-\beta }\]


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