When the answer key says "Omitted", it means the problem is not asking for short answers (whether numerical or not); it would be a proof. Proofs are not included as part of the answer key.  For the Example Questions sections, the proofs are included in the full solution part of the book.  The Practice Problems sections do not provide full solutions, so any question should be  posted here and we will help.  If it is a case not covered here, please indicate the course full name, chapter in the book, and question number, so the teachers can take a look.