Yuánshì

Yuanshi lies beneath Fenglong Mountain at the edge of the Hebei plain. Buddhist caves on the mountain contain carved images made over centuries, from Northern Qi through Tang and Song; an inscription on the cliff dates from the Ming. The mountain also provided a retreat for the mathematician Li Ye, who wrote the Sea-Mirror of Circle Measurement in 1248. His work used algebraic methods to solve problems involving circles and right triangles.