During the February 26th MoMath Masters Tournament, @MoMath1 tweeted, “No googling – how many sides on an enneagon?” We thought, “Hey! We know enneagons!” If you don’t, maybe this scenario from a problem we first used in 1998 will give you some hints (as well as some ideas for something you could do with one).

## Extending the Enneagon

Extend the sides AB and ED of the regular enneagon ABCDEFGHI until they intersect.

I noticed that the enneagon has 9 vertices ABCDEFGHI. I noticed that when you extend two sides of the shape, they will intersect. I wonder how many sides an enneagon has?