Math is often about finding the right analogy, often a spatial analogy. Suppose you want to compute the sum of integers, $latex S = 1+2+3+ \hdots + N$. Consider decomposing the sum as,
$latex \begin{array}{cccccc}S = & 1 & + 1 & + 1 & + \hdots & + 1\\ & & + 1 & + 1 & + \hdots & + 1 \\ & & & + 1 & + \hdots & + 1\\ & & & & \vdots & \\ & & & & & + 1,\end{array}$
where you’ll note the column sums equal the integers in the sequence. You’ll have a bunch of ones that form a triangle. Imagine taking such triangle, copying it, flipping it and then joining the copy to the first triangle. Removing the “$latex +$” signs you’d get something that looks like,
$latex \begin{array}{ccccc} 1 & 1 & 1 & \hdots & 1\\ (1) & 1 & 1 & \hdots & 1\\ (1) & (1) & 1 & \hdots & 1\\ & & & \vdots & \\ (1) & (1) & (1) & \hdots & 1 \\ (1) & (1) & (1) & \hdots & (1), \end{array}$
where I’ve put parentheses on the $latex 1$’s from the second, copied triangle. By analogy, the sum of the integers from the original sequence is equal to half the area of a rectangle that is characterized by this matrix of ones—that is, a rectangle of height $latex n+1$ and width $latex n$. As such, $latex S = n(n+1)/2$. This comes up, e.g., in the asymptotic approximation for the Wilcoxon signed rank test (link).
