Question 467012: sum of 1+2+3+...+81
Answer by Alan3354(69443)   (Show Source):
You can put this solution on YOUR website! You can add them with a calculator, might take awhile.
You can have Excel do it, quicker and less chance of error.
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There are 81 elements.
If you make pairs with the outside elements, 1 & 81, 2 & 80, etc, the pairs add to 82.
You'll get 40 pairs and the middle number left over. The middle number is 41
40*82 + 41 = 3321
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