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How do I calculate the sum of 1+2+3+..up to 50? I knew the answer to this at one point in time but have since forgotten the formula.

I0_ol
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    Add last number to first number. Multiply result by the number of numbers. (Can you figure out how many numbers there are to be summed?) Divide the result by 2. For more reading, take a look at this. – Frenzy Li Aug 30 '16 at 06:11
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    Hint: This is the subject of a famous anecdote about young Gauss and busywork assigned by a teacher. Although not an exact duplicate, the "trick" is fully explained by the answers to this older Question. – hardmath Aug 30 '16 at 19:02

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$1+2+\cdots+n = \frac{n(n+1)}{2}$

user288742
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