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