4thtriangularnumber

<-- Enter triangular number statement
  

Calculate triangular number 4

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

Given n = 4, we have:
T4  =  4(4 + 1)
  2

T4  =  4(5)
  2

T4  =  20
  2

T4 = 10