Is it appropriate to give feedback to instructor about final exam? I wrote a final exam in a course, in which the professor said that he would not test our memory and give all the needed formulae, so I didn't concentrate on memorizing them.

Would it be appropriate to politely send the professor an email with some feedback / opinion about this situation? And if this would be appropriate, what would be the best way to express this opinion as neutrally and politely as possible? So that the professor wouldn't think that I'm mad at him or something like that.
The personalities of professors vary wildly, so you may want to first consider if your professor would be open to feedback about the exam. Also, make sure that you aren't the only or one of a few students who made such mistakes. For instance, is it possible that you missed some content or communication about the exam? If you are on the same page as others, you have a stronger case.

If you feel like giving feedback is beneficial, make sure that you express it constructively and professionally. Some professors may dismiss feedback that simply comes across as "This wasn't fair." For instance, explaining that "many students who studied were confused by some of the communication about how to approach the exam. Is it possible to have a review sheet provided for the next exam?" Or "I studied and followed instructions on the exam, but am confused about how I missed some things that were discussed before the exam. Can you help me understand where I went wrong or how to better prepare next time?" This is less confrontational and helps open a difficult dialogue.

I hope this helps!