A goat is tied to an external corner of a rectangular shed measuring 20m by 15m. If the goat's rope is 13m long, what is the total area of grass in square meters, the goat can graze on?
If you draw a picture of this region, we see that we get $3/4$ of a circle of radius $13$.
The physical reasoning behind this can be seen if we think about how the rope is being used in this sense. Tie it to one end and stretch it out to its max length, then if we want to keep the rope tight, we can only move in a circle. And since the rope is $13$ meters long while the sides of the shed are $15$ and $20$ meters, we cannot go around another corner of the shed.