12.1 problem Ex 1
  12.2 problem Ex 2
  12.3 problem Ex 3
  12.4 problem Ex 4
  12.5 problem Ex 5
  12.6 problem Ex 6
  12.7 problem Ex 7
  12.8 problem Ex 8
  12.9 problem Ex 10
  12.10 problem Ex 11
  12.11 problem Ex 12
  12.12 problem Ex 13
  12.13 problem Ex 14
  12.14 problem Ex 15
  12.15 problem Ex 16
  12.16 problem Ex 17
  12.17 problem Ex 18
  12.18 problem Ex 19
  12.19 problem Ex 20
  12.20 problem Ex 21
  12.21 problem Ex 22
  12.22 problem Ex 23
  12.23 problem Ex 24
  12.24 problem Ex 25
  12.25 problem Ex 26
  12.26 problem Ex 27
  12.27 problem Ex 28
  12.28 problem Ex 29
  12.29 problem Ex 30