1.1 problem 6
 1.2 problem 7
 1.3 problem 8
 1.4 problem 9
 1.5 problem 10
 1.6 problem 11
 1.7 problem 12
 1.8 problem 13
 1.9 problem 14
 1.10 problem 16
 1.11 problem 17
 1.12 problem 18
 1.13 problem 19