Railways Sample Paper RRB (Group D) Sample Test Paper-15

  • question_answer
    If \[{{(2a-\,1)}^{2}}+{{(4b-\,3)}^{2}}+{{(4c\text{ }+\text{ }5)}^{2}}=0\] then the value of \[\frac{{{a}^{3}}+{{b}^{3}}+{{c}^{3}}-\,3abc}{{{a}^{2}}+{{b}^{2}}+{{c}^{2}}}\,\,is:\]

    A)  \[1\frac{3}{8}\]            

    B)  \[3\frac{3}{8}\]

    C)  \[2\frac{3}{8}\]            

    D)  0

    Correct Answer: D

    Solution :

    [d] \[{{(2a-\,1)}^{2}}+{{(4b-\,3)}^{2}}+{{(4c+5)}^{2}}=0\] \[\begin{matrix}    =2a-1=0; & 4b-3=0; & 4c+5=0  \\    a=\frac{1}{2} & b=\frac{3}{4}; & c=\frac{-5}{4}  \\ \end{matrix}\] \[{{a}^{3}}+{{b}^{3}}+{{c}^{3}}-\,3\,\,abc\]\[=(a+b+c)\,\,({{a}^{2}}+{{b}^{2}}+{{c}^{2}}-\,ab-\,bc-\,ca)\]\[But\,\,a+b+c=0\] \[So,\,\,{{a}^{3}}+{{b}^{3}}+{{c}^{3}}-\,3\,\,abc=0\] \[So,\,\,\frac{{{a}^{3}}+{{b}^{3}}+{{c}^{3}}-\,3\,\,abc}{{{a}^{3}}+{{b}^{3}}+{{c}^{3}}}=0\]


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