This makes me wonder about the times I found problems with the answers in the back of the book. It is incredibly frustrating to work on a math problem for two o

This makes me wonder about the times I found problems with the answers in the back of the book. It is incredibly frustrating to work on a math problem for two or three hours, look in the back of the book to see if your answer is right, only to find it wasn't, and then to subsequently find out (from the professor) that the provided answer is wrong.