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Easy

Question

In a sequence of (4n + 1) terms the first (2n + 1) terms are in AP whose common difference is 2 and the last (2n + 1) terms are in GP whose common ratio is 0.5. If the middle terms of the AP and GP are equal then the middle term of the sequence is

  1. fraction numerator n times 2 to the power of n plus 1 end exponent over denominator 2 to the power of n end exponent minus 1 end fraction    
  2. fraction numerator n times 2 to the power of n plus 1 end exponent over denominator 2 to the power of 2 n end exponent minus 1 end fraction    
  3. n times 2 to the power of n end exponent    
  4. text None of these end text    

The correct answer is: fraction numerator n times 2 to the power of n plus 1 end exponent over denominator 2 to the power of n end exponent minus 1 end fraction


    text Let the sequence is  end text a comma a plus 2 comma a plus 4 comma horizontal ellipsis comma a plus 4 n text  , end text
    fraction numerator a plus 4 n over denominator 2 end fraction comma fraction numerator a plus 4 n over denominator 4 end fraction comma midline horizontal ellipsis comma fraction numerator a plus 4 n over denominator 2 to the power of 2 n end exponent end fraction
    It is given that
    a plus 2 n equals fraction numerator a plus 4 n over denominator 2 to the power of n end exponent end fraction
    rightwards double arrow a open parentheses 1 minus fraction numerator 1 over denominator 2 to the power of n end exponent end fraction close parentheses equals fraction numerator 4 n over denominator 2 to the power of n end exponent end fraction minus 2 n rightwards double arrow a equals fraction numerator 4 n minus 2 n times 2 to the power of n end exponent over denominator 2 to the power of n end exponent minus 1 end fraction
    text Therefore, middle term is  end text a plus 4 n equals fraction numerator 2 n minus open parentheses 2 minus 2 to the power of n end exponent close parentheses over denominator 2 to the power of n end exponent minus 1 end fraction plus 4 n equals fraction numerator 2 n times 2 to the power of n end exponent over denominator 2 to the power of n end exponent minus 1 end fraction

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