Grazing Goat

A shed is in a large field. The shed has dimensions 8m x 4m. A goat is grazing in the field (outside the shed) constrained by a tether of length 10m attached to the midpoint of one of the longer sides of the shed. What is the area of the field accessible to the goat? Source: this is a classic problem drawn from Professor Higgins's Problem Collection by Peter M. Higgins (2017, OUP, p. 55). Variations: the shed dimensions and the length of the tether can be adjusted by moving the red points; the position of the goat can also be adjusted (by moving the dotted line).