VIT Engineering VIT Engineering Solved Paper-2008

  • question_answer
    Let \[\alpha ,\beta \] be the roots of the equation \[{{x}^{2}}-ax+b=0\] and \[{{A}_{n}}={{\alpha }^{n}}+{{\beta }^{n}}.\] Then, \[{{A}_{n+1}}-a{{A}_{n}}+b{{A}_{n-1}}\] is equal to

    A)  \[-a\]           

    B)  \[b\]

    C)  0             

    D)  \[a-b\]

    Correct Answer: C

    Solution :

    \[\because \]\[\alpha \] and \[\beta \] be the roots of\[{{x}^{2}}-ax+b=0\]. \[\therefore \]\[{{\alpha }^{2}}-a\alpha +b=0\] and \[{{\beta }^{2}}-\alpha \beta +b=0\] Now, \[{{A}_{n+1}}-a{{A}_{n}}+b{{A}_{n-1}}\] \[={{\alpha }^{n+1}}+{{\beta }^{n+1}}-\alpha ({{\alpha }^{n}}+{{\beta }^{n}})+b({{\alpha }^{n-1}}+{{\beta }^{n-1}})\] \[={{\alpha }^{n-1}}({{\alpha }^{2}}-a\alpha +b)+{{\beta }^{n-1}}({{\beta }^{2}}-a\beta +b)\] \[=0\]


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