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
 1.16 problem 16
 1.17 problem 17
 1.18 problem 18
 1.19 problem 19