12.1 problem 1
12.2 problem 2
12.3 problem 3
12.4 problem 4
12.5 problem 5
12.6 problem 6
12.7 problem 7
12.8 problem 8
12.9 problem 9
12.10 problem 10
12.11 problem 11
12.12 problem 12(a)
12.13 problem 12(b)
12.14 problem 13
12.15 problem 14
12.16 problem 15
12.17 problem 16
12.18 problem 17