2.1 problem 1
2.2 problem 2
2.3 problem 3
2.4 problem 4
2.5 problem 5
2.6 problem 6
2.7 problem 7
2.8 problem 8
2.9 problem 9
2.10 problem 11
2.11 problem 12
2.12 problem 13
2.13 problem 14
2.14 problem 15
2.15 problem 16
2.16 problem 20
2.17 problem 21
2.18 problem 22
2.19 problem 23