Question 11037: What is the correct way to find the sum of 1+4+7+10+13......100
Answer by longjonsilver(2297)   (Show Source):
You can put this solution on YOUR website! one formula is  where a is first term and l is last term.
Issue is, how many terms?, well 3,6,9,...,102 is the 3-times table with 34 terms.
1,4,7,...,100 is the same set of numbers, shunted down by 2... so 34 terms.
so, 
jon.
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