[Maurice Tift]

Sometimes your answer doesn't match the answer in the back of the book.

Backs of books are written by people and people are no good.

[Crolis]

I had a professor for linear algebra who had an interesting comment about the back of the book. He said many of the student assistants who are assisting the professor who is authoring the book try to come up with the most compact way to express the answer. Which is why in many cases the answer you come up with looks nothing like what is printed in the book.