2.1 problem 1
 2.2 problem 2
 2.3 problem 3
 2.4 problem 4
 2.5 problem 5
 2.6 problem 6
 2.7 problem 7
 2.8 problem 8
 2.9 problem 9
 2.10 problem 11
 2.11 problem 12
 2.12 problem 13
 2.13 problem 14
 2.14 problem 15
 2.15 problem 16
 2.16 problem 20
 2.17 problem 21
 2.18 problem 22
 2.19 problem 23