SSC Quantitative Aptitude Algebra Question Bank Elementary Algebra (I)

  • question_answer
    If \[3x+\frac{1}{2x}=5,\]then the value of \[8{{x}^{3}}+\frac{1}{27{{x}^{3}}}\] is

    A) (a)\[118\frac{1}{2}\]

    B) \[30\frac{10}{27}\]

    C) \[0\]

    D) \[1\]

    Correct Answer: B

    Solution :

    [b] \[3x+\frac{1}{2x}=5\] \[\Rightarrow \]   \[\frac{3}{2}\left( 2x+\frac{1}{3x} \right)=5\] \[\Rightarrow \]   \[2x+\frac{1}{3x}=\frac{10}{3}\]                                  …(i) \[\Rightarrow \]   \[{{\left( 2x+\frac{1}{3x} \right)}^{3}}=\frac{1000}{27}\] \[\Rightarrow \]   \[8{{x}^{3}}+\frac{1}{27{{x}^{3}}}+3\times 2x\times \frac{1}{3x}\left( 2x\frac{1}{3x} \right)\] \[=\frac{1000}{27}\] \[\Rightarrow \]   \[8{{x}^{3}}+\frac{1}{27{{x}^{3}}}+2\times \left( \frac{10}{3} \right)=\frac{1000}{27}\] [from Eq. (i)] \[\Rightarrow \]\[8{{x}^{3}}+\frac{1}{27{{x}^{3}}}=\frac{1000}{27}-\frac{20}{3}=\frac{820}{27}=30\frac{10}{27}\]


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