From, well, the tail of an ox. It couldn't be any simpler. Quora probably will tell me this will collapse because it's too short, so: it's very common in Brazil's countryside. It's fat (too fat for me, not my favourite food), but healthy.
We can divide octagon into six triangles. So, the sum of the angles of each triangle is 180 degrees. For octagon, the sum of the interior angles is 1080 degrees and there are eight sides. Let us make a circle in the middle of the octagon and measure the central angle of a regular octagon. A circle is 360 degrees around.
What is the length of the Apothem of a regular octagon with side of length a. The perpendicular distance from the center of a polygon to one of its sides is the Apothem. The slice of one-eighth of the octagon is an isosceles triangle with base of length a and height of the Apothem. The central angle is 45 degrees. Set up a right triangle and use the tangent function.