The tether problem is a classic problem from grade school mathematics. A cow is tied to a rectangular building with a leash of fixed length. For given dimensions of the rectangle and length of leash, the student must determine the area of the region the cow is free to graze. The solution involves sectors of circles whose radii depend on where the leash wraps around the corner of the building.

If the building is a silo (i.e. a cylinder), the problem is more difficult. The boundary of the region includes a portion of the involute of a circle, and the area can be determined using multivariate calculus [1]. In this Demonstration (inspired by Archimedes's approximation of

using regular polygons with more and more sides), we show how the elementary approach involving sectors can be extended to the circle case.

Suppose the base of the building is a regular

-gon with semiperimeter

and that the length of the leash is also

. Focusing on the top half of the polygon and starting opposite to where the leash is tethered, going clockwise, let the vertices be

, where

. For

, the area

of the colored sector at

depends on the measure of the external angle

and the radius at that vertex

;

. The sum of the areas of the colored sectors is

as

. The area of the gray sector is

as

. As

, the area of the sectors converges to

. By symmetry, the total area the cow is free to roam is twice this, or

.