CEE Kerala Engineering CEE Kerala Engineering Solved Paper-2010

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

    A)  \[ac{{x}^{2}}-bx+1=0\]

    B)  \[{{x}^{2}}-acx+bc+1=0\]

    C)  \[ac{{x}^{2}}+bx-1=0\]

    D)  \[{{x}^{2}}+acx-bc+11=0\]

    E)  \[ac{{x}^{2}}-bx-11=0\]

    Correct Answer: A

    Solution :

    \[\because \] \[\alpha ,\beta \]are roots of equation, \[a{{x}^{2}}+bx+c=0\]                            \[(c\ne 0)\] \[\therefore \]  \[\alpha +\beta =-\frac{b}{a},\alpha \beta =\frac{c}{a}\] \[\therefore \]Required equation is \[{{x}^{2}}-\left( \frac{1}{a\alpha +b}+\frac{1}{\alpha \beta +b} \right)x+\left( \frac{1}{a\alpha +b}.\frac{1}{a\beta +b} \right)\] \[\Rightarrow \] \[{{x}^{2}}-\left( \frac{a(\alpha +\beta )+2b}{{{a}^{2}}\alpha \beta +ab(\alpha +\beta )+{{b}^{2}}} \right)x\]                              \[+\left( \frac{1}{{{a}^{2}}\alpha \beta +ab(\alpha +\beta )+{{b}^{2}}} \right)=0\] \[\Rightarrow \]               \[{{x}^{2}}-\left( \frac{a\left( -\frac{b}{a} \right)+2b}{{{a}^{2}}.\frac{c}{a}+ab\left( -\frac{b}{a} \right)+{{b}^{2}}} \right)x\]                 \[+\frac{1}{{{a}^{2}}.\frac{c}{a}+ab\left( -\frac{b}{a} \right)+{{b}^{2}}}=0\] \[\Rightarrow \]               \[{{x}^{2}}-\frac{b}{ac}x+\frac{1}{ac}=0\] \[\Rightarrow \]               \[ac{{x}^{2}}-bx+1=0\]


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