Hard MCQ +4 / -1 PYQ · JEE Mains 2021

The sum of the series

$${1 \over {x + 1}} + {2 \over {{x^2} + 1}} + {{{2^2}} \over {{x^4} + 1}} + ...... + {{{2^{100}}} \over {{x^{{2^{100}}}} + 1}}$$ when x = 2 is :

  1. A $1 + {{{2^{101}}} \over {{4^{101}} - 1}}$
  2. B $1 + {{{2^{100}}} \over {{4^{101}} - 1}}$
  3. C $1 - {{{2^{100}}} \over {{4^{100}} - 1}}$
  4. D $1 - {{{2^{101}}} \over {{2^{400}} - 1}}$ Correct answer

Solution

$$S = {1 \over {x + 1}} + {2 \over {{x^2} + 1}} + {{{2^2}} \over {{x^4} + 1}} + ...... + {{{2^{100}}} \over {{x^{{2^{100}}}} + 1}}$$<br><br>$$S + {1 \over {1 - x}} = {1 \over {1 - x}} + {1 \over {x + 1}} + ...... = {2 \over {1 - {x^2}}} + {2 \over {1 + {x^2}}} + ....$$<br><br>$S + {1 \over {1 - x}} = {{{2^{101}}} \over {1 - {x^{400}}}}$<br><br>$S = 1 - {{{2^{101}}} \over {{2^{400}} - 1}}$

About this question

Subject: Mathematics · Chapter: Sequences and Series · Topic: Arithmetic Progression

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