13.1 problem 1
13.2 problem 2
13.3 problem 4
13.4 problem 5
13.5 problem 6
13.6 problem 7
13.7 problem 9
13.8 problem 10
13.9 problem 11
13.10 problem 12
13.11 problem 13
13.12 problem 15
13.13 problem 16
13.14 problem 17
13.15 problem 18
13.16 problem 21
13.17 problem 23
13.18 problem 24
13.19 problem 25
13.20 problem 26
13.21 problem 27
13.22 problem 28