JEE Main & Advanced Mathematics Binomial Theorem and Mathematical Induction Question Bank Binomial theorem for any index

  • question_answer
    If x is so small that \[{{x}^{3}}\] and higher powers of x may be neglected, then \[\frac{{{(1+x)}^{\frac{3}{2}}}-{{\left( 1+\frac{1}{2}x \right)}^{3}}}{{{(1-x)}^{\frac{1}{2}}}}\] may be approximated as [AIEEE 2005]

    A) \[-\frac{3}{8}{{x}^{2}}\]

    B) \[\frac{x}{2}-\frac{3}{8}{{x}^{2}}\]

    C) \[1-\frac{3}{8}{{x}^{2}}\]

    D) \[3x+\frac{3}{8}{{x}^{2}}\]

    Correct Answer: A

    Solution :

    \[\frac{{{\left( 1+x \right)}^{3/2}}-{{\left( 1+\frac{1}{2}x \right)}^{3}}}{{{\left( 1-x \right)}^{1/2}}}\] = \[\frac{1+\frac{3}{2}x+\frac{\frac{3}{2}.\frac{1}{2}}{2}{{x}^{2}}-\left( 1+\frac{3x}{2}+\frac{3.2}{2}\frac{{{x}^{2}}}{4} \right)}{{{\left( 1-x \right)}^{1/2}}}\] = \[\frac{-\frac{3}{8}{{x}^{2}}}{{{\left( 1-x \right)}^{1/2}}}\] = \[-\frac{3}{8}{{x}^{2}}{{\left( 1-x \right)}^{-1/2}}\] = \[-\frac{3}{8}{{x}^{2}}\left( 1+\frac{x}{2}+.... \right)\] = \[-\frac{3}{8}{{x}^{2}}\].


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