12.1 problem 1
  12.2 problem 2
  12.3 problem 3
  12.4 problem 4
  12.5 problem 5
  12.6 problem 6
  12.7 problem 7
  12.8 problem 8
  12.9 problem 9
  12.10 problem 10
  12.11 problem 11
  12.12 problem 12(a)
  12.13 problem 12(b)
  12.14 problem 13
  12.15 problem 14
  12.16 problem 15
  12.17 problem 16
  12.18 problem 17