There are 8 radials in the octagon so the angle between 2 will be 360/8= 45
The octagon is made up of 8 isosceles triangles. If the vertex of the isosceles triangle is 45 then the angle opposite the two side will be (180 -45) /2 =67.5
Sorry if it seems confusing. I had a similar question in my notebook so I looked off that. I hope this helps and I apologize if it's wrong!