triangular numbers
nounUK /traɪˈæŋɡjʊlə ˈnʌmbəz/
US /traɪˈæŋɡjələr ˈnʌmbərz/
Triangular numbers are numbers you can make by arranging dots in a triangle, like 1, 3, 6, and 10.
The first four triangular numbers are 1, 3, 6, and 10.
OTHER MEANINGS
2. Numbers of the form n(n+1)/2, the sum of the first n positive integers.
The tenth triangular number is 55.
3. A sequence appearing in counting arrangements of objects in an equilateral triangle.
Triangular numbers show up when stacking bowling pins.
4. Values found along the third diagonal of Pascal's triangle.
You can read triangular numbers from Pascal's triangle.