12.1 problem Ex 1
 12.2 problem Ex 2
 12.3 problem Ex 3
 12.4 problem Ex 4
 12.5 problem Ex 5
 12.6 problem Ex 6
 12.7 problem Ex 7
 12.8 problem Ex 8
 12.9 problem Ex 10
 12.10 problem Ex 11
 12.11 problem Ex 12
 12.12 problem Ex 13
 12.13 problem Ex 14
 12.14 problem Ex 15
 12.15 problem Ex 16
 12.16 problem Ex 17
 12.17 problem Ex 18
 12.18 problem Ex 19
 12.19 problem Ex 20
 12.20 problem Ex 21
 12.21 problem Ex 22
 12.22 problem Ex 23
 12.23 problem Ex 24
 12.24 problem Ex 25
 12.25 problem Ex 26
 12.26 problem Ex 27
 12.27 problem Ex 28
 12.28 problem Ex 29
 12.29 problem Ex 30