Question 472301
Yes, you do apply those theorems. However I have a feeling you are applying those theorems without enough conceptual understanding of them (e.g. how are they derived?), in which it diminishes the quality of learning.


Deriving the polygon angle-sum theorem is quite straightforward. Given an n-gon, you can always draw n-2 non-overlapping triangles within it (fix a vertex, connect it with two adjacent vertices). The sum of the interior angles in a triangle is 180, so the sum of all the interior angles is 180(n-2).


There are n straight angles in the n-gon so there are 180n degrees total (part a).


Since the interior and exterior angle add up to a straight angle, we subtract the sum of the interior angles from 180n, e.g. 180n - 180(n-2) = 360, the sum of the exterior angles.