Down here you will find the answers (PDF) to the extra exercises.
Answers exercises chapter 1
Answers exercises chapter 2
Answers exercises chapter 3
Answers exercises chapter 4
Answers exercises chapter 5
Answers exercises chapter 6
Answers exercises chapter 7
Answers exercises chapter 8
Answers exercises chapter 9
Answers exercises chapter 10
Answers exercises chapter 11
Answers exercises chapter 12