4.1 problem 1
4.2 problem 2
4.3 problem 3
4.4 problem 4
4.5 problem 5
4.6 problem 6
4.7 problem 7
4.8 problem 8
4.9 problem 9
4.10 problem 10
4.11 problem 12
4.12 problem 13
4.13 problem 14
4.14 problem 15
4.15 problem 16
4.16 problem 17
4.17 problem 18
4.18 problem 19
4.19 problem 20