13.1 problem 1
  13.2 problem 2
  13.3 problem 3
  13.4 problem 4
  13.5 problem 5
  13.6 problem 6
  13.7 problem 7
  13.8 problem 8
  13.9 problem 9
  13.10 problem 10
  13.11 problem 11
  13.12 problem 12
  13.13 problem 13
  13.14 problem 14
  13.15 problem 15
  13.16 problem 19
  13.17 problem 20
  13.18 problem 21
  13.19 problem 22
  13.20 problem 23
  13.21 problem 24
  13.22 problem 25
  13.23 problem 26
  13.24 problem 27
  13.25 problem 28
  13.26 problem 29
  13.27 problem 31(a)
  13.28 problem 31(b)
  13.29 problem 31(c)
  13.30 problem 31(d)
  13.31 problem 31(e)
  13.32 problem 32
  13.33 problem 33
  13.34 problem 34
  13.35 problem 35
  13.36 problem 36
  13.37 problem 37
  13.38 problem 38
  13.39 problem 39 (a)
  13.40 problem 39 (b)
  13.41 problem 40
  13.42 problem 41
  13.43 problem 42
  13.44 problem 43
  13.45 problem 44
  13.46 problem 45
  13.47 problem 46
  13.48 problem 47
  13.49 problem 48
  13.50 problem 49