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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)
81
×
79
(2)
2
68
(3)
2
67
(4)
69
×
71
(5)
28
×
32
(6)
21
×
19
(7)
21
×
19
(8)
2
78
(9)
2
29
(10)
2
43
(11)
83
×
77
(12)
83
×
77
©2026 数学クラブ http://sugaku.club/
月 日( )
【解答例】
(1)
81
×
79
2
2
=
(
80
+
1
)
(
80
−
1
)
=
80
−
1
=
6400
−
1
=
6399
(2)
2
68
2
2
2
=
(
70
−
2
)
=
70
−
2
×
70
×
2
+
2
=
4900
−
280
+
4
=
4624
(3)
2
67
2
2
2
=
(
70
−
3
)
=
70
−
2
×
70
×
3
+
3
=
4900
−
420
+
9
=
4489
(4)
69
×
71
2
2
=
(
70
−
1
)
(
70
+
1
)
=
70
−
1
=
4900
−
1
=
4899
(5)
28
×
32
2
2
=
(
30
−
2
)
(
30
+
2
)
=
30
−
2
=
900
−
4
=
896
(6)
21
×
19
2
2
=
(
20
+
1
)
(
20
−
1
)
=
20
−
1
=
400
−
1
=
399
(7)
21
×
19
2
2
=
(
20
+
1
)
(
20
−
1
)
=
20
−
1
=
400
−
1
=
399
(8)
2
78
2
2
2
=
(
80
−
2
)
=
80
−
2
×
80
×
2
+
2
=
6400
−
320
+
4
=
6084
(9)
2
29
2
2
2
=
(
30
−
1
)
=
30
−
2
×
30
×
1
+
1
=
900
−
60
+
1
=
841
(10)
2
43
2
2
2
=
(
40
+
3
)
=
40
+
2
×
40
×
3
+
3
=
1600
+
240
+
9
=
1849
(11)
83
×
77
2
2
=
(
80
+
3
)
(
80
−
3
)
=
80
−
3
=
6400
−
9
=
6391
(12)
83
×
77
2
2
=
(
80
+
3
)
(
80
−
3
)
=
80
−
3
=
6400
−
9
=
6391