If a dectagon is another word for a decagon then this is a 10 sided polygon.
No matter how many sides it has, the sum of the external angles of a convex or regular polygon is 360°.
Each exterior angle measures 36 degrees
Providing that it is a regular octagon each exterior angle measures 45 degrees.
Each exterior angle measures 8 degrees
72 degrees
Each exterior angle measures 10 degrees Each interior angle measures 170 degrees
Each exterior angle measures 72 degrees
Each exterior angle measures 72 degrees
Each exterior angle of a regular 6 sided hexagon measures 60 degrees
Each exterior angle of a regular octagon measures 45 degrees
Each exterior angle measures 15 degrees
Each exterior angle measures 36 degrees
Each exterior angle measures 40 degrees
Providing that it is a regular duodecagon each exterior angle measures 30 degrees
Exterior angle measures 18 degrees Interior angle measures 162 degrees
180
45 degrees
If you mean a regular pentagon then each interior angle is 108 degrees and each exterior angle is 72 degrees