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