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This question already has an answer here:

Why does $1+2+\dots+2003=\dfrac{2004\cdot2003}2$?

Sorry if this is missing context; not really much to add...

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marked as duplicate by Stefan Perko, suomynonA, Community Dec 24 '16 at 23:44

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  • $\begingroup$ have you heard arithmetic progression? en.wikipedia.org/wiki/Arithmetic_progression $\endgroup$ – duanduan Dec 24 '16 at 23:28
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    $\begingroup$ Because half a square is a triangle. $\endgroup$ – Evariste Dec 24 '16 at 23:31
  • $\begingroup$ @Evariste what? $\endgroup$ – suomynonA Dec 24 '16 at 23:32
  • $\begingroup$ @Evariste isn't it a rectangle? $\endgroup$ – Jorge Fernández Hidalgo Dec 25 '16 at 0:17
  • $\begingroup$ I realize I was a bit quick, but essentially the idea is a square. As you can see, $2003\times 2004$ is very close to a square. This pictures helps the understanding a bit :oi67.tinypic.com/e9vvi8.jpg $\endgroup$ – Evariste Dec 25 '16 at 0:28
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$$\begin{array}{ccc} S&=&1&+&2&+&3&+&\ldots&+&2001&+&2002&+&2003\\ S&=&2003&+&2002&+&2001&+&\ldots&+&3&+&2&+&1\\ \hline 2S&=&2004&+&2004&+&2004&+&\ldots&+&2004&+&2004&+&2004 \end{array}$$

There are $2003$ columns, so $2S=2003\cdot2004$, and therefore $S=\dfrac{2003\cdot2004}2$.

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    $\begingroup$ This formatting is gorgeous; great use of array. $\endgroup$ – Omnomnomnom Dec 24 '16 at 23:33
  • $\begingroup$ @suomynonA: You’re welcome! $\endgroup$ – Brian M. Scott Dec 24 '16 at 23:34
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By symmetry, the numbers are all centered around $\frac{n+1}{2}$, and there are $n$ of them.

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  • $\begingroup$ Yeah, the number $\frac{n+1}{2}$ is an axis of symmetry for the numbers, when you look at them on the number line. $\endgroup$ – Jorge Fernández Hidalgo Dec 24 '16 at 23:32
  • $\begingroup$ This kind of answer is only useful to those who are used to phrases like "by symmetry" $\endgroup$ – Omnomnomnom Dec 24 '16 at 23:32
  • $\begingroup$ well. I tried to answer the question "why" as much as possible. $\endgroup$ – Jorge Fernández Hidalgo Dec 24 '16 at 23:33
  • $\begingroup$ Oh, I see, thanks $\endgroup$ – suomynonA Dec 24 '16 at 23:33
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$$\sum_{i=1}^n i = \frac{n(n+1)}{2}$$

You can show by induction.

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