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