Done exercise4.25、4.26
Done exercise4.23
Done exercise4.28、4.30
Done exercise4.31、4.32、4.33、4.34、4.35
Done exercise4.36
Done exercise5.1、5.2、5.3、5.4
Done exercise5.7、5.8
Done exercise5.10
Done exercise5.18
Done exercise5.20、5.21
Done exercise5.22、5.23