Triangular Number: Figurate number
A triangular number is a number that is the sum of all of the natural numbers up to a certain number.
When formed using regularly spaced dots, they tend to form a shape of equilateral triangle, hence the name.
For example, 10 is a "triangular number" because .
The first 25 triangular numbers are: 0, 1, 3, 6, 10, 15, 21, 28, 36, 45, 55, 66, 78, 91, 105, 120, 136, 153, 171, 190, 210, 231, 253, 276, 300, 325, 351, and so on.
A triangular number is calculated by the equation: .
References
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