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Question

The sum to n terms of the series 6 times 9 plus 12 times 21 plus 20 times 37 plus 30 times 57 plus 42 times 81 plus midline horizontal ellipsis is

  1. fraction numerator 2 over denominator 5 end fraction n left parenthesis n plus 1 right parenthesis left parenthesis n plus 2 right parenthesis left parenthesis n plus 3 right parenthesis left parenthesis n plus 4 right parenthesis plus fraction numerator 1 over denominator 3 end fraction left parenthesis n plus 1 right parenthesis left parenthesis n plus 2 right parenthesis left parenthesis n plus 3 right parenthesis    
  2. fraction numerator 2 over denominator 5 end fraction n left parenthesis n plus 1 right parenthesis left parenthesis n plus 2 right parenthesis left parenthesis n plus 3 right parenthesis left parenthesis n plus 4 right parenthesis minus fraction numerator 1 over denominator 3 end fraction left parenthesis n plus 1 right parenthesis left parenthesis n plus 2 right parenthesis left parenthesis n plus 3 right parenthesis    
  3. fraction numerator 2 over denominator 5 end fraction n left parenthesis n minus 1 right parenthesis left parenthesis n minus 2 right parenthesis left parenthesis n minus 3 right parenthesis left parenthesis n minus 4 right parenthesis plus fraction numerator 1 over denominator 3 end fraction left parenthesis n minus 1 right parenthesis left parenthesis n minus 2 right parenthesis left parenthesis n minus 3 right parenthesis    
  4. None of these    

The correct answer is: fraction numerator 2 over denominator 5 end fraction n left parenthesis n plus 1 right parenthesis left parenthesis n plus 2 right parenthesis left parenthesis n plus 3 right parenthesis left parenthesis n plus 4 right parenthesis plus fraction numerator 1 over denominator 3 end fraction left parenthesis n plus 1 right parenthesis left parenthesis n plus 2 right parenthesis left parenthesis n plus 3 right parenthesis


    612203042 comma horizontal ellipsis comma n to the power of text th  end text end exponent text  term  end text equals n to the power of 2 end exponent plus 3 n plus 2
    equals left parenthesis n plus 1 right parenthesis left parenthesis n plus 2 right parenthesis
    921375781 comma horizontal ellipsis comma n to the power of text th  end text end exponent text  term  end text equals 2 n to the power of 2 end exponent plus 6 n plus 1
    equals 2 n left parenthesis n plus 3 right parenthesis plus 1
    text Therefore,  end text u subscript n end subscript equals left parenthesis n plus 1 right parenthesis left parenthesis n plus 2 right parenthesis left curly bracket 2 n left parenthesis n plus 3 right parenthesis plus 1 right curly bracket
    equals 2 n left parenthesis n plus 1 right parenthesis left parenthesis n plus 2 right parenthesis left parenthesis n plus 3 right parenthesis plus left parenthesis n plus 1 right parenthesis left parenthesis n plus 2 right parenthesis
    rightwards double arrow S subscript n end subscript equals fraction numerator 2 over denominator 5 end fraction n left parenthesis n plus 1 right parenthesis left parenthesis n plus 2 right parenthesis left parenthesis n plus 3 right parenthesis left parenthesis n plus 4 right parenthesis plus fraction numerator 1 over denominator 3 end fraction left parenthesis n plus 1 right parenthesis left parenthesis n plus 2 right parenthesis left parenthesis n plus 3 right parenthesis

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