Tetrahedral numbers

http://mathworld.wolfram.com/TetrahedralNumber.html

Triangular Number

http://mathworld.wolfram.com/TriangularNumber.html

Polygonal Number

http://mathworld.wolfram.com/PolygonalNumber.html

Tetrahedral numbers have the form n(n+1)(n+2)/6 and

triangular numbers have the form n(n+1)/2.

Show that at least one-third of the tetrahedral numbers are also polygonal numbers.