3.1 problem 1
 3.2 problem 2
 3.3 problem 3
 3.4 problem 4
 3.5 problem 5
 3.6 problem 6
 3.7 problem 11
 3.8 problem 12
 3.9 problem 13
 3.10 problem 14
 3.11 problem 15
 3.12 problem 16
 3.13 problem 17
 3.14 problem 18
 3.15 problem 19
 3.16 problem 20