unknown
it would have 18 triangles because u take 2 away each time.
It can be divided into 18, hope this helps!
The number of triangles formed by the diagonals of a 21 sided polygon is C(21,3) = 21x20x19/3x2x1 = 1,330.
18. If a polygon has n sides, then it has n-2 triangles.