2.1 problem 1
  2.2 problem 2
  2.3 problem 3
  2.4 problem 4
  2.5 problem 5
  2.6 problem 6
  2.7 problem 7
  2.8 problem 8
  2.9 problem 9
  2.10 problem 11
  2.11 problem 12
  2.12 problem 13
  2.13 problem 14
  2.14 problem 15
  2.15 problem 16
  2.16 problem 20
  2.17 problem 21
  2.18 problem 22
  2.19 problem 23