4.1 problem 84
  4.2 problem 87
  4.3 problem 90
  4.4 problem 91
  4.5 problem 92
  4.6 problem 94
  4.7 problem 95
  4.8 problem 96
  4.9 problem 98
  4.10 problem 99
  4.11 problem 100
  4.12 problem 101
  4.13 problem 102
  4.14 problem 103
  4.15 problem 104
  4.16 problem 105
  4.17 problem 106
  4.18 problem 107
  4.19 problem 108
  4.20 problem 109
  4.21 problem 110
  4.22 problem 111
  4.23 problem 112
  4.24 problem 113
  4.25 problem 114
  4.26 problem 115