23.1 problem 1
23.2 problem 2
23.3 problem 3
23.4 problem 4
23.5 problem 5
23.6 problem 6
23.7 problem 7
23.8 problem 8
23.9 problem 9
23.10 problem 10
23.11 problem 15
23.12 problem 16
23.13 problem 17
23.14 problem 18
23.15 problem 19
23.16 problem 20
23.17 problem 21
23.18 problem 22
23.19 problem 23
23.20 problem 24
23.21 problem 25
23.22 problem 26