Khối 7
Toán 7 - Cơ bản và nâng cao - Tập một
Chương I SỐ HỮU TỈ
§2. Cộng, trừ số hữu tỉ
Mức độ nâng cao
Ví dụ
5
∗
~5^{*}
5
∗
. Tính:
A
=
7
3.4
−
9
4.5
+
11
5.6
−
13
6.7
+
15
7.8
−
17
8.9
+
19
9.10
A=\dfrac{7}{3.4}-\dfrac{9}{4.5}+\dfrac{11}{5.6}-\dfrac{13}{6.7}+\dfrac{15}{7.8}-\dfrac{17}{8.9}+\dfrac{19}{9.10}
A
=
3.4
7
−
4.5
9
+
5.6
11
−
6.7
13
+
7.8
15
−
8.9
17
+
9.10
19
.
Bài giải
A
=
6
+
1
12
−
10
−
1
20
+
10
+
1
30
−
14
−
1
42
+
14
+
1
56
−
18
−
1
72
+
18
+
1
90
=
6
12
+
1
12
−
10
20
+
1
20
+
10
30
+
1
30
−
14
42
+
1
42
+
14
56
+
1
56
−
18
72
+
1
72
+
18
90
+
1
90
=
1
2
+
1
12
−
1
2
+
1
20
+
1
3
+
1
30
−
1
3
+
1
42
+
1
4
+
1
56
−
1
4
+
1
72
+
1
5
+
1
90
=
1
12
+
1
20
+
1
30
+
1
42
+
1
56
+
1
72
+
1
90
+
1
5
=
1
3
−
1
4
+
1
4
−
1
5
+
1
5
−
1
6
+
1
6
−
1
7
+
1
7
−
1
8
+
1
8
−
1
9
+
1
9
−
1
10
+
1
5
=
1
3
−
1
10
+
1
5
=
10
30
−
3
30
+
6
30
=
13
30
.
\begin{aligned} A &=\dfrac{6+1}{12}-\dfrac{10-1}{20}+\dfrac{10+1}{30}-\dfrac{14-1}{42}+\dfrac{14+1}{56}-\dfrac{18-1}{72}+\dfrac{18+1}{90} \\ &=\dfrac{6}{12}+\dfrac{1}{12}-\dfrac{10}{20}+\dfrac{1}{20}+\dfrac{10}{30}+\dfrac{1}{30}-\dfrac{14}{42}+\dfrac{1}{42}+\dfrac{14}{56}+\dfrac{1}{56}-\dfrac{18}{72}+\dfrac{1}{72}+\dfrac{18}{90}+\dfrac{1}{90} \\ &=\dfrac{1}{2}+\dfrac{1}{12}-\dfrac{1}{2}+\dfrac{1}{20}+\dfrac{1}{3}+\dfrac{1}{30}-\dfrac{1}{3}+\dfrac{1}{42}+\dfrac{1}{4}+\dfrac{1}{56}-\dfrac{1}{4}+\dfrac{1}{72}+\dfrac{1}{5}+\dfrac{1}{90} \\ &=\dfrac{1}{12}+\dfrac{1}{20}+\dfrac{1}{30}+\dfrac{1}{42}+\dfrac{1}{56}+\dfrac{1}{72}+\dfrac{1}{90}+\dfrac{1}{5} \\ &=\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{10}+\dfrac{1}{5} \\ &=\dfrac{1}{3}-\dfrac{1}{10}+\dfrac{1}{5}=\dfrac{10}{30}-\dfrac{3}{30}+\dfrac{6}{30}=\dfrac{13}{30} . \end{aligned}
A
=
12
6
+
1
−
20
10
−
1
+
30
10
+
1
−
42
14
−
1
+
56
14
+
1
−
72
18
−
1
+
90
18
+
1
=
12
6
+
12
1
−
20
10
+
20
1
+
30
10
+
30
1
−
42
14
+
42
1
+
56
14
+
56
1
−
72
18
+
72
1
+
90
18
+
90
1
=
2
1
+
12
1
−
2
1
+
20
1
+
3
1
+
30
1
−
3
1
+
42
1
+
4
1
+
56
1
−
4
1
+
72
1
+
5
1
+
90
1
=
12
1
+
20
1
+
30
1
+
42
1
+
56
1
+
72
1
+
90
1
+
5
1
=
3
1
−
4
1
+
4
1
−
5
1
+
5
1
−
6
1
+
6
1
−
7
1
+
7
1
−
8
1
+
8
1
−
9
1
+
9
1
−
10
1
+
5
1
=
3
1
−
10
1
+
5
1
=
30
10
−
30
3
+
30
6
=
30
13
.
Đáp án
1/1