10th Class Mathematics Related to Competitive Exam Question Bank Algebra

  • question_answer
    If \[\alpha ,\beta \] are the roots of the equation \[\alpha {{x}^{2}}+bx+c=\]0, then the value of \[\frac{1}{a\alpha +b}+\frac{1}{a\beta +b}\]is

    A)  \[\frac{b}{ac}\]                 

    B)         \[\frac{a}{bc}\]

    C)  \[\frac{c}{ab}\]               

    D)         \[\frac{bc}{a}\]

    Correct Answer: A

    Solution :

     Since \[\alpha \] and \[\beta \] are the roots of the equation,                 \[a{{x}^{2}}+bx+c=0\] \[\therefore \]  \[\alpha +\beta =-\frac{b}{a}\]and\[\alpha \beta =\frac{c}{a}\] Now\[\frac{1}{a\alpha +b}+\frac{1}{a\beta +b}=\frac{a\beta +b+a\beta +b}{{{a}^{2}}\alpha \beta +ab(\alpha +\beta )+{{b}^{2}}}\]                 \[=\frac{a\left( -\frac{b}{a} \right)+2b}{{{a}^{2}}\cdot \frac{c}{a}+ab\left( -\frac{b}{a} \right)+{{b}^{2}}}=\frac{b}{ac}\]


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