ADAM HART-DAVIS / SCIENCE PHOTO LIBRARY ADAM HART-DAVIS / SCIENCE PHOTO LIBRARY
Triangular numbers. Stones laid out in a triangle to represent the sequence of numbers formed by the successive addition of consecutive numbers, of which the first term and common difference are 1. Therefore, triangular numbers are 1 (first row), 3 (first and second row), 6 (first, second and third row), 10 (first, second, third and fourth row) and so on.
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