For examples (similar goat grazing) click here:
The area the goat can graze - Google Search=
For examples (similar goat grazing) click here:
The area the goat can graze - Google Search=
Let be the area the goat can graze in a circle around the pen from when the rope (fully extended) is paralel to BC until it is parallel to AB.
Let be the remaining quarter circile the goat can graze after its tether makes contact with point A.
Then the area the goat can graze is given by: