1.1 problem 1.1
  1.2 problem 1.2
  1.3 problem 1.3
  1.4 problem 1.4
  1.5 problem 1.5
  1.6 problem 1.6
  1.7 problem 3.1
  1.8 problem 3.2
  1.9 problem 3.3
  1.10 problem 3.4
  1.11 problem 3.5
  1.12 problem 4.1
  1.13 problem 4.2
  1.14 problem 6.1
  1.15 problem 6.2
  1.16 problem 6.3
  1.17 problem 6.4
  1.18 problem 6.5
  1.19 problem 10.1
  1.20 problem 10.2
  1.21 problem 10.3
  1.22 problem 10.4
  1.23 problem 10.5