JEE Main & Advanced Mathematics Sequence & Series Question Bank Self Evaluation Test - Sequences and Series

  • question_answer
    If \[\left| x \right|<\frac{1}{2},\] what is the value of \[1+n\left[ \frac{x}{1-x} \right]+\left[ \frac{n(n+1)}{2!} \right]{{\left[ \frac{x}{1-x} \right]}^{2}}+.......\infty \]?

    A) \[{{\left[ \frac{1-x}{1-2x} \right]}^{n}}\]

    B) \[{{(1-x)}^{n}}\]

    C) \[{{\left[ \frac{1-2x}{1-x} \right]}^{n}}\]

    D) \[{{\left( \frac{1}{1-x} \right)}^{n}}\]

    Correct Answer: A

    Solution :

    [a] Given that \[1+n\left| \frac{x}{1-x} \right|+\frac{n(n+1)}{2!}{{\left| \frac{x}{1-x} \right|}^{2}}\] \[+....\infty \] is expansion of \[{{\left| 1-\frac{x}{1-x} \right|}^{-n}}\]. So, it is \[={{\left| 1-\frac{x}{1-x} \right|}^{-n}}\] \[={{\left| \frac{1-x-x}{1-x} \right|}^{-n}}={{\left| \frac{1-x}{1-2x} \right|}^{n}}\]


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