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 16(a)
  13.17 problem 16(b)
  13.18 problem 17
  13.19 problem 18
  13.20 problem 19(a)
  13.21 problem 19(b)
  13.22 problem 19(c)
  13.23 problem 20