1, the chapter arrangement is different. The last chapter of the fifth edition is arranged to the last chapter of the sixth edition. 2. The exercises are different. Although the fifth edition is very similar to the sixth edition, there are still some differences. But it doesn't matter if you want to buy second-hand books. Depending on the typography, the exercise questions are different, but generally you don't do the questions in the book in class. I am a sophomore. I started with the fifth edition, no problem.