Maths-
General
Easy

Question

The sum of the series 1 times n plus 1 plus 2 times open parentheses n minus 1 close parentheses plus 3 times open parentheses n minus 2 close parentheses horizontal ellipsis plus n times 1 is

  1. fraction numerator n open parentheses n plus 1 close parentheses left parenthesis n plus 2 right parenthesis over denominator 6 end fraction    
  2. fraction numerator n open parentheses n plus 1 close parentheses open parentheses n plus 2 close parentheses over denominator 3 end fraction    
  3. fraction numerator n open parentheses n plus 1 close parentheses open parentheses 3 n plus 2 close parentheses over denominator 6 end fraction    
  4. fraction numerator n open parentheses n plus 1 close parentheses open parentheses 2 n plus 2 close parentheses over denominator 3 end fraction    

The correct answer is: fraction numerator n open parentheses n plus 1 close parentheses left parenthesis n plus 2 right parenthesis over denominator 6 end fraction


    1 bullet n plus 2 open parentheses n minus 1 close parentheses plus 3 open parentheses n minus 2 close parentheses plus... plus n bullet 1
    equals not stretchy sum from r equals 1 to n of open parentheses n plus 1 close parentheses r minus not stretchy sum from r equals 1 to n of r to the power of 2 end exponent
    equals open parentheses n plus 1 close parentheses not stretchy sum n minus not stretchy sum n to the power of 2 end exponent
    equals fraction numerator open parentheses n plus 1 close parentheses n left parenthesis n plus 1 right parenthesis over denominator 2 end fraction minus fraction numerator n open parentheses n plus 1 close parentheses open parentheses 2 n plus 1 close parentheses over denominator 6 end fraction
    equals fraction numerator n open parentheses n plus 1 close parentheses over denominator 6 end fraction open curly brackets 3 n plus 3 minus 2 n minus 1 close curly brackets
    equals fraction numerator n open parentheses n plus 1 close parentheses left parenthesis n plus 2 right parenthesis over denominator 6 end fraction

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