First of all, the cowhide area of a bull is certain. If you enclose the land with cowhide and cut it into strings, you can enclose much more land than covering it with cowhide.

Second, save some cowhide by the seaside, so that the saved cowhide can enclose more land.

Third, Dido's circle is a semicircle. Among all the shapes, the area of the circle is the largest when the circumference is fixed, because it is close to the sea, which saves the length of the coastline. So circle into a semicircle, the largest area.

So, she bent the rope into a semicircle at the seaside and circled a considerable piece of land with the sea as the boundary.