Tetrahedral Number Sequence
The Tetrahedral Number Sequence can be easily understood if you think of a stack of marbles in the shape of a Tetrahedron.
Just count how many marbles would be needed for a stack of a certain height.
Triangular and Tetrahedral Numbers
This table shows the values for the first few layers:
|n||Triangular Number||Tetrahedral Number|
|(Height)||(Marbles in Layer)||(Total Marbles)|
If you look at the numbers you can see something interesting: if you take any number and add the number below and to the left, you get the next number in the sequence. (For example 6+4=10).