Clever visual proof

There are as many yellow dots above the bottom row of the triangle as there are pairs of purple dots on the bottom row. To see this, note that every yellow dot determines a pair of purple dots by projecting it down to the left and to the right. Conversely, you can go up from any pair of purple dots up to a yellow dot.

Via sark

Related post: Tetrahedral numbers

9 thoughts on “Clever visual proof

  1. Nice! Perhaps leave out ‘in the top of the triangle’ or replace it with ‘above the bottom row’.

  2. I didn’t understand how the sequence-sum explains the pair-counting property.
    BTW, doesn’t sequence sum equals N*(N-1)/2?

  3. Regarding triangular numbers, it makes perfect sense sense as the number of dots above the bottom row is simply the previous triangular number.

  4. If you add an upside down copy of the yellow triangle next to the original one to form a parallelogram, then you can also see at a glance that the there are n*(n-1)/2 yellow dots.

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