9thpentagonalnumber

<-- Enter pentagonal number statement
  

Calculate pentagonal number 9

The formula for pentagonal number n is as follows
Pn  =  n(3n - 1)
  2

Given n = 9, we have:
P9  =  9(3(9) - 1)
  2

P9  =  9(27 - 1)
  2

P9  =  9(26)
  2

P9  =  234
  2

P9 = 117