hard implicit function theorem

例句：

1. 1. The material is hard to work with.
   这种材料很难加工。
2. 2. He works very hard to achieve his goals.
   他非常努力地实现他的目标。
3. 3. The car was hard to start in the cold weather.
   这辆车在寒冷的天气里很难启动。