かけ算21の段(21の倍数)

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

20 × 11 = 220

を計算して 11 を足します。

21の段

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

ピックアップ

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

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