The ( n^{th} ) triangular number ( T_n ) counts the number of dots in an equilateral triangle arranged in ( n ) rows:
[ T_n = 1 + 2 + \cdots + n = \dfrac{n(n+1)}{2} ]
The ( n^{th} ) triangular number ( T_n ) counts the number of dots in an equilateral triangle arranged in ( n ) rows:
[ T_n = 1 + 2 + \cdots + n = \dfrac{n(n+1)}{2} ]