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Nhớ rằng: \(\frac{n}{a.\left(a+n\right)}=\frac{1}{a}+\frac{1}{a+n}\)
Ta có: \(A=\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+...+\frac{1}{90}\)
\(A=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{9.10}\)
Áp dụng công thức trên, ta có:
\(A=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{9}-\frac{1}{10}\)
\(A=1-\frac{1}{10}=\frac{9}{10}\)
Trả lời :
= 7
Còn cách làm tham khảo Kiều Anh nha
#Mập
Odd one out
1. A.scarf B.sandals C.boots D.shoes
2. A.soccer B.tennis C.badminton D.sport
3. A.elephant B.animal C.dolphin D.shark
4. A.play B.seafood C.sandcastle D.beach ball
5.A.violin B.guitar C.music D.piano
6. A.football B.table tennis C.volleyball D.play
7. A.music B.english C.books D.physics
8. A. party B. invite C. complete D. money câu này chịu
9. A.flowers B. visit C. clothes D.sport câu này chịu
10. A. their B. me C. his D. our
11. A. coat B. sandals C. boots D. shoes
11.A.cycle B.ski C.skate D.chess
12.A. giraffe B. seafood C. dolphin D. shark
13.A.play B. badminton C.football D. chess
14. A.english B.music C.subject D.history
nếu sai cho mình xin lỗi
1. A.scarf B.sandals C.boots D.shoes
2. A.soccer B.tennis C.badminton D.sport
3. A.elephant B.animal C.dolphin D.shark
4. A.play B.seafood C.sandcastle D.beach ball
5.A.violin B.guitar C.music D.piano
6. A.football B.table tennis C.volleyball D.play
7. A.music B.english C.books D.physics
8. A. party B. invite C. complete D. money
9. A.flowers B. visit C. clothes D.sport
10. A. their B. me C. his D. our
11. A. coat B. sandals C. boots D. shoes
11.A.cycle B.ski C.skate D.chess
12.A. giraffe B. seafood C. dolphin D. shark
13.A.play B. badminton C.football D. chess
14. A.english B.music C.subject D.history
I. PHONICS
1.A.this
2.A. small
3 B.door
4. C.who
5 D. flower
6.D.handsome
7. D.watch
8.A.brother
9. D. fall
10.A.nose B.flower C.hold D. cold
I. PHONICS
1.A.this B.thin C.thick D.thanks
2.A. small B. tall C.bad D.ball
3.A. room B.door C.school D.book
4.A.when B.why C.who D.where
5.A.nose B.cold C. hold D. flower
6.A. large B. bad C.fat D.handsome
7.A. Bay B. play C.stay D.watch
8.A.brother B.open C.so D.cold
9. A. take B. late C. table D. fall
10.A.nose B.flower C.hold D. cold
* Nếu sai thì thông cảm ạ :) *
1. Where do your mother often buy this book? => Where does your mother often buy this book?
2. Are Tom studying? => Is Tom studying?
3. Hoa not have good speaking skill. => Hoa does not have good speaking skill
4. The young girl not writing the letter now. => The young girl is not writing the letter now.
5. Are Lam fond of listening music?=> Is Lam fond of listening to music?
6. They studies math=> They study math
7. What you usually do in the morning? => What do you usually do in the morning?
8. They are traveling to London. => Th ey are travelling to London.
9. The plane is land now.=> The plane is landing now
10. Why she is saying loudly?=> Why is she saying loudly?
1. Where do your mother often buy this book?
=> Where does your mother often buy this book?
2. Are Tom studying English now?
=>Is tom studying English now?
3. Hoa not have good speaking skill.
=>Hoa doesn't have good speaking skill.
4. The young girl not writing the letter now.
=> That young girl isn't writing the letter now.
5. Are Lam fond of listening music?
=> Does Lam fond of listening to music?
6. They studies math once time a week.
=> They study math once time a week.
7. What you usually do in the morning?
=> What do you usually do in the morning?
8. They are traveling to London.
=>They are travelling to London.
9. The plane is land now.
=> The plane is landing now.
10. Why she is saying loudly?
=> why is she saying loudly?
Hok tốt
21. doesn't see
22. doesn't work
23. is riding
24. am speaking
25. are planting
26. Have - take
27.lies - is resting - falls
28. go- visit
29. lives- is studying
30. are expecting- are receiving- is giving
Bài nghe:
1. How can I get to the bookshop?
(Làm thế nào để tôi có thể đến hiệu sách?)
2. How can I get to the campsite?
(Làm thế nào để tôi có thể đến khu cắm trại?)
Lời giải chi tiết:
1. c 2. a
\(A=\frac{1}{3}.\left(\frac{3}{2.4.6}+\frac{3}{4.6.8}+...+\frac{3}{22.24.26}\right)\)
\(A=\frac{1}{3}.\left(\frac{1}{2.4}-\frac{1}{4.6}+\frac{1}{4.6}-\frac{1}{6.8}+...+\frac{1}{22.24}-\frac{1}{24.26}\right)\)
\(A=\frac{1}{3}.\left(\frac{1}{2.4}-\frac{1}{24.26}\right)\)
\(A=\frac{1}{3}.\frac{77}{624}\)
\(A=\frac{77}{1872}\)