Stack dots in a triangle: 1, 3, 6, 10, 15, 21… Each one is the previous plus the next counting number.
nth triangular number
Tₙ = n(n+1)/2
Try it
T₅
5·6/2 = 15. ✓
Stack dots in a triangle: 1, 3, 6, 10, 15, 21… Each one is the previous plus the next counting number.
T₅
5·6/2 = 15. ✓