费马原理帮助我们理解为什么光在水和空气的界面会发生折射。

The ___Fermat principle helps us understand why light refracts at the interface of water and air.

费马大定理是数学史上最著名的未解决问题之一。

___Fermat's Last Theorem is one of the most famous unsolved problems in the history of mathematics.