3.1.1 problem 1
  3.1.2 problem 2
  3.1.3 problem 3
  3.1.4 problem 4
  3.1.5 problem 5
  3.1.6 problem 7
  3.1.7 problem 8
  3.1.8 problem 9
  3.1.9 problem 10
  3.1.10 problem 12
  3.1.11 problem 13
  3.1.12 problem 14