How did Lu Bu, Xia and Guo Jia learn the art of war?

The method is as follows:

1. Books: refer to historical books or talk specifically about the works of Lu Bu, Xia Houyuan and Guo Jia, and learn about the contents and related materials of their art books.

2. Academic research: search for academic research papers, academic journals or academic conferences, consult relevant research materials, and understand scholars' interpretations and research results of The Art of War by Lu Bu, Xia and Guo Jia.

3. Network resources: Using the Internet search engine, you can find some online resources, forum discussions or blog articles by searching the relevant materials of The Art of War by Lu Bu, Xia and Guo Jia.

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