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©2026 数学クラブ http://sugaku.club/
月 日（ ）
● 次の計算をしなさい
[1C1-00]
(1)
21
×
19
(2)
23
×
17
(3)
2
51
(4)
82
×
78
(5)
37
×
43
(6)
2
89
(7)
2
53
(8)
22
×
18
(9)
2
79
(10)
72
×
68
(11)
2
47
(12)
39
×
41
©2026 数学クラブ http://sugaku.club/
月 日（ ）
【解答例】
(1)
21
×
19
2
2
=
(
20
＋
1
)
(
20
−
1
)
=
20
−
1
=
400
−
1
=
399
(2)
23
×
17
2
2
=
(
20
＋
3
)
(
20
−
3
)
=
20
−
3
=
400
−
9
=
391
(3)
2
51
2
2
2
=
(
50
＋
1
)
=
50
＋
2
×
50
×
1
＋
1
=
2500
＋
100
＋
1
=
2601
(4)
82
×
78
2
2
=
(
80
＋
2
)
(
80
−
2
)
=
80
−
2
=
6400
−
4
=
6396
(5)
37
×
43
2
2
=
(
40
−
3
)
(
40
＋
3
)
=
40
−
3
=
1600
−
9
=
1591
(6)
2
89
2
2
2
=
(
90
−
1
)
=
90
−
2
×
90
×
1
＋
1
=
8100
−
180
＋
1
=
7921
(7)
2
53
2
2
2
=
(
50
＋
3
)
=
50
＋
2
×
50
×
3
＋
3
=
2500
＋
300
＋
9
=
2809
(8)
22
×
18
2
2
=
(
20
＋
2
)
(
20
−
2
)
=
20
−
2
=
400
−
4
=
396
(9)
2
79
2
2
2
=
(
80
−
1
)
=
80
−
2
×
80
×
1
＋
1
=
6400
−
160
＋
1
=
6241
(10)
72
×
68
2
2
=
(
70
＋
2
)
(
70
−
2
)
=
70
−
2
=
4900
−
4
=
4896
(11)
2
47
2
2
2
=
(
50
−
3
)
=
50
−
2
×
50
×
3
＋
3
=
2500
−
300
＋
9
=
2209
(12)
39
×
41
2
2
=
(
40
−
1
)
(
40
＋
1
)
=
40
−
1
=
1600
−
1
=
1599