Triangular and Tetrahedral Numers

####Update: Flattened the spheres for easy printing. No raft is necessary now.

Models for triangular numbers {1, 3, 6, 10, 15, ..., n(n+1)/2}. They can also be used to model tetrahedral numbers {1, 4, 10, 20, 35, 56, ..., n (n+1)(n+2)/6}. The spheres are intentionally welded with each other for easy use with students. The activities can be used at many K-12 levels, depending on the instructional goals. The formulas can be established using mathematical induction; they are as interesting as the visual structures. To print, please use raft for better adhesion to the bed.