why 是 Q，不在 the other 侧，如果我们从 where 向下移动 (South)，dh/dx 是 0 than 并且 h which 减少 right？我的意思是 dh/dy < 0 对于 both 点 right？why 仅适用于 the 上方？####[['Q', 'Mathematical_Related_Concept', 'NEU'], ['h', 'Mathematical_Related_Concept', 'NEU'], ['dh/dy < 0', 'Mathematical_Related_Concept', 'NEU'], ['dh/dx', 'Mathematical_Related_Concept', 'NEU']]