18.1 problem 477
 18.2 problem 478
 18.3 problem 479
 18.4 problem 480
 18.5 problem 481
 18.6 problem 482
 18.7 problem 483
 18.8 problem 484
 18.9 problem 485
 18.10 problem 486
 18.11 problem 487
 18.12 problem 488
 18.13 problem 489
 18.14 problem 490
 18.15 problem 491
 18.16 problem 492
 18.17 problem 493
 18.18 problem 494
 18.19 problem 495
 18.20 problem 496
 18.21 problem 497
 18.22 problem 498
 18.23 problem 499
 18.24 problem 500
 18.25 problem 501
 18.26 problem 502
 18.27 problem 505
 18.28 problem 506
 18.29 problem 507
 18.30 problem 508
 18.31 problem 509
 18.32 problem 510
 18.33 problem 511
 18.34 problem 512
 18.35 problem 513