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