3rdtriangularnumber

<-- Enter triangular number statement
  

Calculate triangular number 3

The formula for triangular number n is as follows
Tn  =  n(n + 1)
  2

Given n = 3, we have:
T3  =  3(3 + 1)
  2

T3  =  3(4)
  2

T3  =  12
  2

T3 = 6