7thpentagonalnumber

<-- Enter pentagonal number statement
  

Calculate pentagonal number 7

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

Given n = 7, we have:
P7  =  7(3(7) - 1)
  2

P7  =  7(21 - 1)
  2

P7  =  7(20)
  2

P7  =  140
  2

P7 = 70