19.1 problem 1
 19.2 problem 2
 19.3 problem 3
 19.4 problem 4
 19.5 problem 5
 19.6 problem 6
 19.7 problem 7
 19.8 problem 8
 19.9 problem 9
 19.10 problem 10
 19.11 problem 11
 19.12 problem 12
 19.13 problem 13
 19.14 problem 14
 19.15 problem 15
 19.16 problem 16
 19.17 problem 17
 19.18 problem 18
 19.19 problem 19
 19.20 problem 20
 19.21 problem 21
 19.22 problem 22
 19.23 problem 23
 19.24 problem 24
 19.25 problem 25
 19.26 problem 26
 19.27 problem 27
 19.28 problem 28
 19.29 problem 29
 19.30 problem 30
 19.31 problem 31
 19.32 problem 32
 19.33 problem 33