21の性質
かけ算21の段（21の倍数）
- 21 × 1 = 21
- 21 × 2 = 42
- 21 × 3 = 63
- 21 × 4 = 84
- 21 × 5 = 105
- 21 × 6 = 126
- 21 × 7 = 147
- 21 × 8 = 168
- 21 × 9 = 189
- 21 × 10 = 210
- 21 × 11 = 231
- 21 × 12 = 252
- 21 × 13 = 273
- 21 × 14 = 294
- 21 × 15 = 315
- 21 × 16 = 336
- 21 × 17 = 357
- 21 × 18 = 378
- 21 × 19 = 399
- 21 × 20 = 420
- 21 × 21 = 441
- 21 × 22 = 462
- 21 × 23 = 483
- 21 × 24 = 504
- 21 × 25 = 525
- 21 × 26 = 546
- 21 × 27 = 567
- 21 × 28 = 588
- 21 × 29 = 609
- 21 × 30 = 630
- 21 × 31 = 651
- 21 × 32 = 672
- 21 × 33 = 693
- 21 × 34 = 714
- 21 × 35 = 735
- 21 × 36 = 756
- 21 × 37 = 777
- 21 × 38 = 798
- 21 × 39 = 819
- 21 × 40 = 840
- 21 × 41 = 861
- 21 × 42 = 882
- 21 × 43 = 903
- 21 × 44 = 924
- 21 × 45 = 945
- 21 × 46 = 966
- 21 × 47 = 987
- 21 × 48 = 1008
- 21 × 49 = 1029
- 21 × 50 = 1050
- 21 × 51 = 1071
- 21 × 52 = 1092
- 21 × 53 = 1113
- 21 × 54 = 1134
- 21 × 55 = 1155
- 21 × 56 = 1176
- 21 × 57 = 1197
- 21 × 58 = 1218
- 21 × 59 = 1239
- 21 × 60 = 1260
- 21 × 61 = 1281
- 21 × 62 = 1302
- 21 × 63 = 1323
- 21 × 64 = 1344
- 21 × 65 = 1365
- 21 × 66 = 1386
- 21 × 67 = 1407
- 21 × 68 = 1428
- 21 × 69 = 1449
- 21 × 70 = 1470
- 21 × 71 = 1491
- 21 × 72 = 1512
- 21 × 73 = 1533
- 21 × 74 = 1554
- 21 × 75 = 1575
- 21 × 76 = 1596
- 21 × 77 = 1617
- 21 × 78 = 1638
- 21 × 79 = 1659
- 21 × 80 = 1680
- 21 × 81 = 1701
- 21 × 82 = 1722
- 21 × 83 = 1743
- 21 × 84 = 1764
- 21 × 85 = 1785
- 21 × 86 = 1806
- 21 × 87 = 1827
- 21 × 88 = 1848
- 21 × 89 = 1869
- 21 × 90 = 1890
- 21 × 91 = 1911
- 21 × 92 = 1932
- 21 × 93 = 1953
- 21 × 94 = 1974
- 21 × 95 = 1995
- 21 × 96 = 2016
- 21 × 97 = 2037
- 21 × 98 = 2058
- 21 × 99 = 2079
- 21 × 100 = 2100
21 の倍数は 20 の倍数を考えるとわかりやすい。例えば
21 × 11 = 231
は
20 × 11 = 220
を計算して 11 を足します。
21 × 19 = 399
これは因数分解
$(20 + 1)(20 - 1) = 400 - 1$
からも計算できます。
問題
21 × 17 =
21 × 26 =
21 × 9 =
21 × 23 =
21 × 11 =
解答
21 × 17 = 357
21 × 26 = 546
21 × 9 = 189
21 × 23 = 483
21 × 11 = 231
