Measure it. There is no formula for an exterior angle unless you have a regular (or equiangular) polygon. And there is no evidence to suggest that that is the case.
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It is: 180-interior angle = exterior angle
With a regular polygon: 360/exterior angle = number of sides
Interior and adjacent exterior angles form a straight line so exterior angle = 180 - interior angle. If you are satisfied with this reply please consider awarding a Recommended Point to the responder.
The sum of the exterior angles in any polygon is 360 degrees. A nonagon has 9 sides, so to work out a single exterior angle, we divide 360 by 9, which is 40.
Each exterior angle 45 degrees Each interior angle 135 degrees