Exer. 21.1
Exer. 21.2
Exer. 21.3
Exer. 21.4
Exer. 21.5
Exer. 21.6
Exer. 21.6 another solution
Exer. 21.7
Exer. 21.8
Exer. 21.9
Exer. 21.10
Exer. 21.11
Exer. 21.12
Exer. 21.13
Exer. 21.14
Exer. 21.15
Exer. 21.16