1.1 problem 6
  1.2 problem 7
  1.3 problem 8
  1.4 problem 9
  1.5 problem 10
  1.6 problem 11
  1.7 problem 12
  1.8 problem 13
  1.9 problem 14
  1.10 problem 16
  1.11 problem 17
  1.12 problem 18
  1.13 problem 19