Pentagonal numbers

Triangular number

http://en.wikipedia.org/wiki/Triangular_number
Pentagonal number

http://en.wikipedia.org/wiki/Pentagonal_number

Prove that every pentagonal number greater than one can be written as the sum of three triangular numbers, two of which are equal.

Triangular numbers T_n are positive integers of the form n(n + 1)/2

Pentagonal numbers P_n are positive integers of the form n(3n - 1)/2


@matthen2 posted a picture proof http://twitpic.com/4li960