SOLUTION: 1+2+3...+81= solve it

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Question 1093154: 1+2+3...+81= solve it
Found 2 solutions by ikleyn, Alan3354:
Answer by ikleyn(52778)   (Show Source): You can put this solution on YOUR website!
.
1 + 2 + 3 + . . . + n = the sum of the first n natural numbers = 

                      = the sum of the first n terms of an Arithmetic progression
                        with the first term 1 and the common difference 1 =

                      = .


In your case n = 81 and the sum is   = 81*41 = 3321.


Answer. 3321.

Solved.

---------------
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    - The proofs of the formulas for arithmetic progressions
    - Problems on arithmetic progressions
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Answer by Alan3354(69443)   (Show Source): You can put this solution on YOUR website!
1+2+3...+81= solve it
1+2+3...+80 --> 80 numbers
Make 40 pairs:
1 + 80 = 81
2 + 79 = 81
etc
40*81 = 3240
---
Add 81
--> 3321
=============
If you prefer remembering rules and formulas:
Sum = n*(n+1)/2 = 81*41
= 3321
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