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

  • question_answer
    \[1+\frac{1}{3}x+\frac{1.4}{3.6}{{x}^{2}}+\frac{1.4.7}{3.6.9}{{x}^{3}}+....\]is equal to

    A) x

    B) \[{{(1+x)}^{1/3}}\]

    C) \[{{(1-x)}^{1/3}}\]

    D) \[{{(1-x)}^{-1/3}}\]

    Correct Answer: D

    Solution :

    Let \[{{(1+y)}^{n}}=1+\frac{1}{3}x+\frac{1.4}{3.6}{{x}^{2}}+\frac{1.4.7}{3.6.9}{{x}^{3}}+....\]                  \[=1+ny+\frac{n(n-1)}{2!}{{y}^{2}}+.....\] Comparing the terms, we get \[ny=\frac{1}{3}x,\frac{n(n-1)}{2!}{{y}^{2}}=\frac{1.4}{3.6}{{x}^{2}}\] Solving,  \[n=-\frac{1}{3},y=-x\]. Hence given series \[={{(1-x)}^{-1/3}}\]


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