9thtriangularnumber

<-- Enter triangular number statement
  

Calculate triangular number 9

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

Given n = 9, we have:
T9  =  9(9 + 1)
  2

T9  =  9(10)
  2

T9  =  90
  2

T9 = 45