1.3.1 problem 23
  1.3.2 problem 26
  1.3.3 problem 27
  1.3.4 problem 30
  1.3.5 problem 32
  1.3.6 problem 35
  1.3.7 problem 36
  1.3.8 problem 37
  1.3.9 problem 38
  1.3.10 problem 39
  1.3.11 problem 40
  1.3.12 problem 41