There is only an air-to-air explanation here, I hope you can understand.

For the sake of explanation, we assume that the upper and lower sides of the building are 20 meters long and the left and right sides are 10 meters wide.

(1) The rope is 8 meters long, and the sheep can only eat 3/4 of the circular area in the upper left corner, that is, 8×8×3×3/4= 144 square meters.

(2) The rope length is 15m.

The rope is longer than10m and less than 20m, and the sheep can bypass the left side of the building by 5m. This can eat 3/4 of the upper left area with a radius of 15m and 1/4 of the lower left area with a radius of 5m, that is

15×15× 3× 3/4+5× 5× 3×1/4 = 525m2.

This rope is 30 meters long.

The rope is longer than 20 meters, which is equal to 30 meters. Sheep can just bypass the lower right corner of the building. In this way, it can eat the 3/4 area with a radius of 30 meters, the 1/4 area with a radius of 20 meters below and the 1/4 circular area with a radius of 10 meters on the right, that is,

30×30×3×3/4+(20×20+ 10× 10)×3× 1/4

=2400 square meters.

It's really inconvenient to come in without a plan.

There may be an error in the calculation. For your reference.