13.1 problem 21
13.2 problem 22
13.3 problem 23
13.4 problem 24
13.5 problem 25
13.6 problem 26
13.7 problem 27
13.8 problem 28
13.9 problem 29
13.10 problem 30
13.11 problem 31
13.12 problem 32
13.13 problem 33
13.14 problem 35
13.15 problem 36
13.16 problem 37
13.17 problem 38