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Question

The sum of 20 terms of a series of which every even term is 2 times the term before it, and every odd term is 3 times the term before it, the first term being unity is

  1. open parentheses fraction numerator 2 over denominator 7 end fraction close parentheses left parenthesis 6 to the power of 10 end exponent minus 1 right parenthesis   
  2. open parentheses fraction numerator 3 over denominator 7 end fraction close parentheses left parenthesis 6 to the power of 10 end exponent minus 1 right parenthesis   
  3. open parentheses fraction numerator 3 over denominator 5 end fraction close parentheses left parenthesis 6 to the power of 10 end exponent minus 1 right parenthesis   
  4. None of these  

The correct answer is: open parentheses fraction numerator 3 over denominator 5 end fraction close parentheses left parenthesis 6 to the power of 10 end exponent minus 1 right parenthesis


    The series is
    1 plus 2 plus 2 cross times 3 plus 2 to the power of 2 end exponent cross times 3 plus 2 to the power of 2 end exponent cross times 3 to the power of 2 end exponent plus 2 to the power of 3 end exponent cross times 3 to the power of 2 end exponent plus horizontal ellipsis to 20 terms
    equals left parenthesis 1 plus 2 cross times 3 plus 2 to the power of 2 end exponent cross times 3 to the power of 2 end exponent plus horizontal ellipsis to 10 terms) plus left parenthesis 2 plus 2 to the power of 2 end exponent cross times 3 plus 2 to the power of 3 end exponent cross times 3 to the power of 2 end exponent plus horizontal ellipsis to 10 terms)
    equals fraction numerator 1 left parenthesis 2 to the power of 10 end exponent 3 to the power of 10 end exponent minus 1 right parenthesis over denominator 6 minus 1 end fraction plus fraction numerator 2 left parenthesis 2 to the power of 10 end exponent 3 to the power of 10 end exponent minus 1 right parenthesis over denominator 6 minus 1 end fraction
    equals open parentheses fraction numerator 3 over denominator 5 end fraction close parentheses left parenthesis 6 to the power of 10 end exponent minus 1 right parenthesis

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