You've got to spend the time to make sure that everything is well defined before you start trying to work out these problem More about the path. There are a couple ways you could go through that pass If I look at this smooth path here.

你要花时间确认,在你开始尝试解题前,切已经被很好地定义,关于路径还有些问题,你可以用两种不同的方式经过这条路径,看这条平滑的路径。

麻省理工公开课 - 热力学与动力学课程节选