28.1 problem 798
28.2 problem 799
28.3 problem 800
28.4 problem 801
28.5 problem 802
28.6 problem 804
28.7 problem 805
28.8 problem 806
28.9 problem 807
28.10 problem 808
28.11 problem 809
28.12 problem 810
28.13 problem 811
28.14 problem 812
28.15 problem 813
28.16 problem 814
28.17 problem 815
28.18 problem 816
28.19 problem 817
28.20 problem 818
28.21 problem 819
28.22 problem 820
28.23 problem 821
28.24 problem 822