Each exterior angle of a regular n-gon is 360 divided by n. The exteriors always add to 360. A regular octagon has exteriors of 45 degrees. Interior sum is a different formula, in polygon interior-angle sum.
Walk the boundary
Turning once around the shape is 360 degrees. Regular means those turns match. Interior and exterior at a vertex are supplementary.
Lines and angles as a set are in lines and angles. The short list is in ACT Math formulas.
The bottom line
Regular exterior = 360 / n. Interiors still use (n − 2) × 180.
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