I thought that a regular 9 sided polygon would have 9 equilateral triangles? but in this case the triangle shown is not equilateral. why can we divide the area of a regular polygon to get the area of this triangle?
Equilateral means all laterals/sides are equal and that is true in an equilateral triangle where all internal angles are 60° hence all sides are equal.
Yes.
40-70-70 internal angle triangle so it cannot be equilateral.
Because there are nine 40-70-70 congruent/equal triangles.