What did the mathematician suggest?

Question

A farmer who just inherited 100 hectares of land was having some issues.
He wanted his new property to be fenced without the need to spend a lot of cash.
He consulted three of his friends - an architect, an engineer and a mathematician.
The architect suggested the most typical solution. Just put fences on the land's perimeter.
The engineer thought that he had a better idea. He suggested that the farmer create a circular fence. It can cover a large part of the land with low cost, but small parts of the land will be left unprotected.
The mathematician laughed. He blurted out his idea. The idea satisfied the first condition, which was cover the land with fences, and it satisfied the second condition to the extreme, which was to spend little money as possible.
However, the farmer scratched his head, and waited for the right time to reject the proposal.
What did the mathematician suggest?