難関校を受験する中学受験生に暗記してほしい計算をまとめました。
平方数（30）
11 × 11 = 121
12 × 12 = 144
13 × 13 = 169
14 × 14 = 196
15 × 15 = 225
16 × 16 = 256
17 × 17 = 289
18 × 18 = 324
19 × 19 = 361
20 × 20 = 400
21 × 21 = 441
22 × 22 = 484
23 × 23 = 529
24 × 24 = 576
25 × 25 = 625
26 × 26 = 676
27 × 27 = 729
28 × 28 = 784
29 × 29 = 841
30 × 30 = 900
31 × 31 = 961
32 × 32 = 1024
33 × 33 = 1089
34 × 34 = 1156
35 × 35 = 1225
36 × 36 = 1296
37 × 37 = 1369
38 × 38 = 1444
39 × 39 = 1521
40 × 40 = 1600
平方数の応用 1（20）
10 × 12 = 120 = 11 × 11 - 1
11 × 13 = 143 = 12 × 12 - 1
12 × 14 = 168 = 13 × 13 - 1
13 × 15 = 195 = 14 × 14 - 1
14 × 16 = 224 = 15 × 15 - 1
15 × 17 = 255 = 16 × 16 - 1
16 × 18 = 288 = 17 × 17 - 1
17 × 19 = 323 = 18 × 18 - 1
18 × 20 = 360 = 19 × 19 - 1
19 × 21 = 399 = 20 × 20 - 1
20 × 22 = 440 = 21 × 21 - 1
21 × 23 = 483 = 22 × 22 - 1
22 × 24 = 528 = 23 × 23 - 1
23 × 25 = 575 = 24 × 24 - 1
24 × 26 = 624 = 25 × 25 - 1
25 × 27 = 675 = 26 × 26 - 1
26 × 28 = 728 = 27 × 27 - 1
27 × 29 = 783 = 28 × 28 - 1
28 × 30 = 840 = 29 × 29 - 1
29 × 31 = 899 = 30 × 30 - 1
99 × 101 = 9999 = 100 × 100 - 1
平方数の応用 2（20）
10 × 14 = 140 = 12 × 12 - 4
11 × 15 = 165 = 13 × 13 - 4
12 × 16 = 192 = 14 × 14 - 4
13 × 17 = 221 = 15 × 15 - 4
14 × 18 = 252 = 16 × 16 - 4
15 × 19 = 285 = 17 × 17 - 4
16 × 20 = 320 = 18 × 18 - 4
17 × 21 = 357 = 19 × 19 - 4
18 × 22 = 396 = 20 × 20 - 4
19 × 23 = 437 = 21 × 21 - 4
20 × 24 = 480 = 22 × 22 - 4
21 × 25 = 525 = 23 × 23 - 4
22 × 26 = 572 = 24 × 24 - 4
23 × 27 = 621 = 25 × 25 - 4
24 × 28 = 672 = 26 × 26 - 4
25 × 29 = 725 = 27 × 27 - 4
26 × 30 = 780 = 28 × 28 - 4
27 × 31 = 857 = 29 × 29 - 4
28 × 32 = 896 = 30 × 30 - 4
11 の倍数（20）
11 × 11 = 121
11 × 12 = 132
11 × 13 = 143
11 × 14 = 154
11 × 15 = 165
11 × 16 = 176
11 × 17 = 187
11 × 18 = 198
11 × 19 = 209
11 × 20 = 220
11 × 21 = 231
11 × 22 = 242
11 × 23 = 253
11 × 24 = 264
11 × 25 = 275
11 × 26 = 286
11 × 27 = 297
11 × 28 = 308
11 × 29 = 319
11 × 30 = 330
12 の倍数（20）
12 × 11 = 132
12 × 12 = 144
12 × 13 = 156
12 × 14 = 168
12 × 15 = 180
12 × 16 = 192
12 × 17 = 204
12 × 18 = 216
12 × 19 = 228
12 × 20 = 240
12 × 21 = 252
12 × 22 = 264
12 × 23 = 276
12 × 24 = 288
12 × 25 = 300
12 × 26 = 312
12 × 27 = 324
12 × 28 = 336
12 × 29 = 348
12 × 30 = 360
15の倍数（20）
15 の倍数は、15 にかける数を半分した値をその数に足して 10 倍すると求まります。
15 × 11 = 165
15 × 12 = 180
15 × 13 = 195
15 × 14 = 210
15 × 15 = 225
15 × 16 = 240
15 × 17 = 255
15 × 18 = 270
15 × 19 = 285
15 × 20 = 300
15 × 21 = 315
15 × 22 = 330
15 × 23 = 345
15 × 24 = 360
15 × 25 = 375
15 × 26 = 390
15 × 27 = 405
15 × 28 = 420
15 × 29 = 435
15 × 30 = 450
25の倍数（20）
25 の倍数は、25 にかける数を 4 で割って 100 倍すると求まります。
25 × 11 = 275
25 × 12 = 300
25 × 13 = 325
25 × 14 = 350
25 × 15 = 375
25 × 16 = 400
25 × 17 = 425
25 × 18 = 450
25 × 19 = 475
25 × 20 = 500
25 × 21 = 525
25 × 22 = 550
25 × 23 = 575
25 × 24 = 600
25 × 25 = 625
25 × 26 = 650
25 × 27 = 675
25 × 28 = 700
25 × 29 = 725
25 × 30 = 750
階乗（5）
1! = 1
2! = 1 × 2 = 2
3! = 1 × 2 × 3 = 6
4! = 1 × 2 × 3 × 4 = 24
5! = 1 × 2 × 3 × 4 × 5 = 120
三角数（15）
ここでは、n 個から k 個を選ぶ組み合わせを n Ｃ k と書きます。受験本番で使わないでください。
n C 2 は特に重要で、三角数と呼ばれます。石を 1、2、3 個と三角形に並べるときの総数です。
2 Ｃ 2 = 1
3 Ｃ 2 = 3
4 Ｃ 2 = 6
5 Ｃ 2 = 10
6 Ｃ 2 = 15
7 Ｃ 2 = 21
8 Ｃ 2 = 28
9 Ｃ 2 = 36
10 Ｃ 2 = 45
11 Ｃ 2 = 55
12 Ｃ 2 = 66
13 Ｃ 2 = 78
14 Ｃ 2 = 91
15 Ｃ 2 = 105
16 Ｃ 2 = 120
円周率（18）
3.14 × 2 = 6.28
3.14 × 3 = 9.42
3.14 × 4 = 12.56
3.14 × 5 = 15.7
3.14 × 6 = 18.84
3.14 × 7 = 21.98
3.14 × 8 = 25.12
3.14 × 9 = 28.26
3.14 × 10 = 31.4
3.14 × 12 = 37.68
3.14 × 15 = 47.1
3.14 × 16 = 50.24
3.14 × 18 = 56.52
3.14 × 20 = 62.8
3.14 × 24 = 75.36
3.14 × 25 = 78.5
3.14 × 30 = 94.2
3.14 × 32 = 100.48
