triangular number in English
noun
1
any of the series of numbers (1, 3, 6, 10, 15, etc.) obtained by continued summation of the natural numbers 1, 2, 3, 4, 5, etc..
This triangle has an elementary use in probability and combinatorics, and both the series of triangular numbers and the Fibonacci series can be derived from it.
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1. For example, the sixth heptagonal number (81) minus the sixth hexagonal number (66) equals the fifth triangular number, 15.