18.1 problem 1
18.2 problem 2
18.3 problem 3
18.4 problem 4
18.5 problem 5 a
18.6 problem 5 c
18.7 problem 6 a
18.8 problem 6 c
18.9 problem 7
18.10 problem 8
18.11 problem 9
18.12 problem 10
18.13 problem 11
18.14 problem 12
18.15 problem 13
18.16 problem 14
18.17 problem 15
18.18 problem 16