12.1 problem 1
 12.2 problem 2
 12.3 problem 3
 12.4 problem 4
 12.5 problem 5
 12.6 problem 6
 12.7 problem 7
 12.8 problem 8
 12.9 problem 9(a)
 12.10 problem 10
 12.11 problem 11
 12.12 problem 12
 12.13 problem 13
 12.14 problem 16
 12.15 problem 17
 12.16 problem 18
 12.17 problem 19
 12.18 problem 20
 12.19 problem 21
 12.20 problem 22
 12.21 problem 23
 12.22 problem 24
 12.23 problem 25
 12.24 problem 26
 12.25 problem 27
 12.26 problem 31
 12.27 problem 33
 12.28 problem 34
 12.29 problem 35
 12.30 problem 36
 12.31 problem 37
 12.32 problem 39
 12.33 problem 40
 12.34 problem 41
 12.35 problem 42
 12.36 problem 43
 12.37 problem 44