Given a regular octagon, find the measures of the angles formed by (a) two consecutive radii and (b) a radius and a side of the polygon.

Well right here the answwer is  45°; 67.5° and let me explain why is it like this: There are 8 radials in the Octagon so the angle between 2 will be
360/8 = 45
Now have in mind that the Octagon is made up of 8 isoceles triangles. So 
If the vertex of the isoceles triangle is 45 then the angle opposite the two equal sides will be
(180 -45) / 2 = 67.5
Hope this is clear and easy to understand