Railways Sample Paper RRB (Group D) Sample Test Paper-4

  • question_answer
    If \[{{a}^{2}}+{{b}^{2}}=5ab\], then the value of \[\left( \frac{{{a}^{2}}}{{{b}^{2}}}+\frac{{{b}^{2}}}{{{a}^{2}}} \right)\]is:

    A)  32                   

    B)  16

    C)  23       

    D)  \[-\,23\]

    Correct Answer: C

    Solution :

    [c] \[{{a}^{2}}+{{b}^{2}}=5ab\] \[\Rightarrow \,\,\,\frac{{{a}^{2}}+{{b}^{2}}}{ab}=5\] \[\Rightarrow \,\,\,\frac{a}{b}+\frac{b}{a}=5\] On squaring both sides. \[\therefore \,\,\,{{\left( \frac{a}{b}+\frac{b}{a} \right)}^{2}}=25\] \[\Rightarrow \,\,\,\frac{{{a}^{2}}}{{{b}^{2}}}+\frac{{{b}^{2}}}{{{a}^{2}}}+2=25\] \[\Rightarrow \,\,\,\frac{{{a}^{2}}}{{{b}^{2}}}+\frac{{{b}^{2}}}{{{a}^{2}}}=25-\,2=23\].


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