19.1 problem 1
19.2 problem 2
19.3 problem 3
19.4 problem 4
19.5 problem 5 a
19.6 problem 5 c
19.7 problem 6 a
19.8 problem 6 c
19.9 problem 7
19.10 problem 8
19.11 problem 9
19.12 problem 10
19.13 problem 11
19.14 problem 12
19.15 problem 13
19.16 problem 14
19.17 problem 15
19.18 problem 16