4.1.1 problem 1
4.1.2 problem 2
4.1.3 problem 3
4.1.4 problem 4
4.1.5 problem 5
4.1.6 problem 6
4.1.7 problem 7
4.1.8 problem 8
4.1.9 problem 9
4.1.10 problem 10
4.1.11 problem 11
4.1.12 problem 12
4.1.13 problem 14
4.1.14 problem 15
4.1.15 problem 16
4.1.16 problem 17