what are the angle on the lines of a octagon
This means that the triangles are all identical, and are the angles (or the triangle) adjacent to the side of the octagon are equal. Since the sum of the angles of a triangle is 180 degrees, each of them measures (180-45)/2 degrees. Since each interior angle of the octagon is comprised of two angles whose measure we just calculated above, each of them measures 180-45 = 135 degrees. It looks as if elbow pieces that measure 45 degrees won't work.