1.1 problem 1
1.2 problem 2
1.3 problem 3
1.4 problem 4
1.5 problem 5
1.6 problem 6
1.7 problem 7
1.8 problem 8
1.9 problem 9
1.10 problem 10
1.11 problem 11
1.12 problem 12
1.13 problem 13
1.14 problem 14
1.15 problem 15