trianglenumber
nounUK /ˈtraɪæŋɡəl ˈnʌmbə/
US /ˈtraɪæŋɡəl ˈnʌmbər/
A triangle number is a number you can make by arranging dots into a triangle shape.
Ten is a triangle number because you can arrange ten dots in a triangle with four rows.
RELATED WORDS
OTHER MEANINGS
2. A number in the sequence 1, 3, 6, 10, 15, ... formed by adding consecutive counting numbers.
The first five triangle numbers are 1, 3, 6, 10, and 15.
3. A number that equals the sum of the first n natural numbers.
Twenty-one is a triangle number because it is the sum of 1 through 6.
4. A number that can be represented by a triangular array of points.
The teacher showed that 28 is a triangle number by drawing a triangle of dots.