The Magic of Maths

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First annotation on Sep 7th, 2025.

We call the numbers 1, 3, 6, 10, and 15 triangular numbers, since we can create triangles like the ones below using those quantities of dots. (You might dispute that 1 dot forms a triangle, but nevertheless 1 is considered triangular.) The official definition is that the nth triangular number is 1 + 2 + 3 + · · · + n.