Triangle numbers are numbers that can perfectly form a triangle, like bowling pins. 10 is a triangle number because it forms the bowling triangle. It’s the intermediate allies when you sum all counting numbers. Short list: - 1 - 3 - 6 - 10 - 15 - 21 - … All numbers can be expressed as the sum of 3 (or fewer) triangle numbers. **** # More ## Source - [[Thinking Better]] ## Related - [[Shape Numbers]]