triangular number

noun

Definition of triangular number 

: a number (such as 3, 6, 10, 15) representable by that many dots arranged in rows that form a triangle and that equals {latex}\frac{n(n + 1)}{2}{/latex} for some positive integer value of n

First Known Use of triangular number

1795, in the meaning defined above

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The first known use of triangular number was in 1795

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Britannica.com: Encyclopedia article about triangular number

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