4.3.1 problem 38
  4.3.2 problem 39
  4.3.3 problem 40
  4.3.4 problem 41
  4.3.5 problem 42
  4.3.6 problem 43
  4.3.7 problem 45
  4.3.8 problem 46
  4.3.9 problem 47
  4.3.10 problem 49
  4.3.11 problem 50
  4.3.12 problem 52
  4.3.13 problem 53
  4.3.14 problem 54
  4.3.15 problem 56
  4.3.16 problem 59
  4.3.17 problem 60