Definition of triangular number
: any of a set of integers (such as 1, 3, 6, 10, 15, ...) each of which represents the total number of dots in a figure formed starting with one dot and adding rows one after another to form a triangle with each row having one more dot than the one before and which equals the sum of the integers from 1 to the number of rows in the triangle
First Known Use of triangular number
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Britannica.com: Encyclopedia article about triangular number
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