Triangular number

Triangular Number Sequence
1, 3, 6, 10, 15, 21, 28, 36, 45, ...

https://upload.wikimedia.org/wikipedia/commons/thumb/1/1c/First_six_triangular_numbers.svg/220px-First_six_triangular_numbers.svg.png

https://wikimedia.org/api/rest_v1/media/math/render/svg/8411fe69f79f2971b7c7a453b0b547bb37e9f6df?fix_wiki_img_ext=.png

https://wikimedia.org/api/rest_v1/media/math/render/svg/c38b21816ac2c7ffc218389b16b4de8ebf8b927d?fix_wiki_img_ext=.png

Tn = n(n+1) / 2

Triangular number - Wikipedia - https://en.wikipedia.org/wiki/Triangular_number

Math is Fun - Triangular Number Sequence - https://www.mathsisfun.com/algebra/triangular-numbers.html

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triangle triangular number sequence