How to avoid plagiarism when including definitions and proofs from textbook?

I need to include some basic definitions, and simple proofs from a standard text book to my thesis. Those definitions and proofs are very basic, e.g. probability, entropy etc. Everybody knows they should be in some text books.

Consider, for example, this proof from wiki. My questions are:

• Is this plagiarism if I bring this whole proof word by word into my thesis?


• If this is plagiarism, what is the method to "paraphrase" a proof? Since the derivation is straightforward, and it is very difficult to write it in a different way?

Thank you.