23.1 problem 631
 23.2 problem 632
 23.3 problem 633
 23.4 problem 634
 23.5 problem 635
 23.6 problem 637
 23.7 problem 638
 23.8 problem 639
 23.9 problem 640
 23.10 problem 641
 23.11 problem 642
 23.12 problem 643
 23.13 problem 644
 23.14 problem 645
 23.15 problem 646
 23.16 problem 647
 23.17 problem 648
 23.18 problem 649
 23.19 problem 650
 23.20 problem 651
 23.21 problem 652
 23.22 problem 653
 23.23 problem 654
 23.24 problem 655
 23.25 problem 656
 23.26 problem 657
 23.27 problem 658
 23.28 problem 659
 23.29 problem 660
 23.30 problem 661
 23.31 problem 662