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4 tháng 9 2021
b: Đặt \(\dfrac{a}{b}=\dfrac{c}{d}=k\)
\(\Leftrightarrow\left\{{}\begin{matrix}a=bk\\c=dk\end{matrix}\right.\)
Ta có: \(\dfrac{\left(a+c\right)^2}{\left(b+d\right)^2}\)
\(=\dfrac{\left(bk+dk\right)^2}{\left(b+d\right)^2}\)
\(=k^2\)(1)
Ta có: \(\dfrac{a^2-c^2}{b^2-d^2}\)
\(=\dfrac{\left(bk\right)^2-\left(dk\right)^2}{b^2-d^2}\)
\(=k^2\left(2\right)\)
Từ (1) và (2) suy ra \(\dfrac{\left(a+c\right)^2}{\left(b+d\right)^2}=\dfrac{a^2-c^2}{b^2-d^2}\)
4 tháng 9 2021
Đặt \(\dfrac{a}{b}=\dfrac{c}{d}=k\)
\(\Leftrightarrow\left\{{}\begin{matrix}a=bk\\c=dk\end{matrix}\right.\)
Ta có: \(\dfrac{4a+3c}{4b+3d}=\dfrac{4bk+3dk}{4b+3d}=k\)
\(\dfrac{4a-3c}{4b-3d}=\dfrac{4bk-3dk}{4b-3d}=k\)
Do đó: \(\dfrac{4a+3c}{4b+3d}=\dfrac{4a-3c}{4b-3d}\)
10 tháng 7 2021
ex 4.
1 seeing
2 to pay
3 sneezing
4 to clean up
5 to work
6 cutting
7 to iron - ironing
8 driving
9 to have
10 to invite
11 being
12 to find
13 holding - drinking
14 to repair
15 explaining
ex 5.
2. Pamela continued dancing for an hour.
3. I'd rather sit in the front row.
4. Richard expects to do well.
5. What do you intend to do for the summer?
6. I hate clearing up my room.
7. Helen agreed to go to the party with me.
8. My boss refused to let me leave early.
9. I really look forward to hearing from you soon.
10. What do you want to do this evening?
11. The manager promised to speak to you about this.
12. Fiona succeeded in persuading her father to change his mind.
13. Where do you feel like going this evening?
14. The detective happened to discover the secret.
15. It was very late, but she went on tidying up the kitchen.
ex6.
1. The room will be cleaned today.
2. The car might be stolen.
3. That tree had to be cut down.
4. The old houses are going to be demolished.
5. The picture can't be restored.
6. An appointment must be made in advance.
7. I don't want to be made a fool.
8. The garden has to be looked after.
9. He wants to be served.
10. He is going to be interviewed next week.
CB
giúp mình vớii ạ:((
1
các bạn ơi giúp mình gấp vớii ạ
11 D
12 B
13 A
14 B
15 A
16 A
17 A
18 B
19 C
20 B
21 B
22 C
23 C
24 C
25 B
26 heated
27 solar
28 pollution
sửa lỗi sài giùm mình với ạ từ 21-25 mình cảm ơn nhiều ạ