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Railways Sample Paper RRB (Group D) Sample Test Paper-2

  • question_answer
    If\[{{x}^{2}}-3x+1=0,\] then what is the value of \[{{x}^{3}}+\frac{1}{{{x}^{3}}}?\]

    A)  3                                

    B)  7

    C)   11                              

    D)  18

    Correct Answer: D

    Solution :

    [d] \[If\,\,{{x}^{2}}-\,3x+1=0\,\,then\,\,{{x}^{3}}+\frac{1}{{{x}^{3}}}=?\] Dividing equation by x \[x-\,3+\frac{1}{x}=0\] \[x+\,\frac{1}{x}=3\] \[\therefore \,\,{{x}^{3}}+\frac{1}{{{x}^{3}}}={{\left( x+\frac{1}{x} \right)}^{3}}-3\,\,\left( x+\frac{1}{x} \right)\] \[={{(3)}^{3}}-\,3\times 3\] \[=27-\,9\] \[=18\]


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