2.1 problem problem 1
2.2 problem problem 2
2.3 problem problem 3
2.4 problem problem 4
2.5 problem problem 5
2.6 problem problem 6
2.7 problem problem 7
2.8 problem problem 8
2.9 problem problem 9
2.10 problem problem 10
2.11 problem problem 11
2.12 problem problem 12
2.13 problem problem 13
2.14 problem problem 14
2.15 problem problem 15
2.16 problem problem 16
2.17 problem problem 17
2.18 problem problem 18
2.19 problem problem 19
2.20 problem problem 20
2.21 problem problem 21
2.22 problem problem 22
2.23 problem problem 23
2.24 problem problem 24
2.25 problem problem 25
2.26 problem problem 26
2.27 problem problem 27
2.28 problem problem 28