SOLUTION: sum the serie: 1+5+9+...+77

Question 945364: sum the serie: 1+5+9+...+77
Answer by josgarithmetic(39618) (Show Source): You can put this solution on YOUR website!
General term of the sequence, for index n.
Which index is the term, 77?

You can avoid dealing with the formula for sum of a sequence if you use simple sense and understanding about arithmetic series.

You have twenty terms. you can form ten pair of sums all equivalent to , or . No term is unpaired because you have n=20, an EVEN number of terms. The sum is 78, occurring in a sum 10 times.

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