B=1+3^2+3^4+3^6+...+3^100

→3^2B=3^2+3^4+3^6+3^8+...+3^102

→3^2B−B=3^102−1

→8B=3^102−1

→B=1/8(3^102−1)

nha bạn 

D = 1+3²+3⁴+3⁶+.......+3¹⁰⁰

=> 3²D = 3²+3⁴+3⁶+.......+3¹⁰⁰+3¹⁰¹

=> 9D - D = (3²+3⁴+3⁶+.......+3¹⁰⁰+3¹⁰¹) - (1+3²+3⁴+3⁶+.......+3¹⁰⁰⁾

=> 8D= 3¹⁰¹ - 1

=> D =  (3¹⁰¹-1) : 8

là biện pháp so sánh có tác dụng làm cho hòn đảo coto thêm sinh động hơn

biện pháp tu từ là so sánh 

a - b = 2(a + b) 

=> a - b = 2a + 2b

<=> a + 3b = 0 

=> a = -3b

=> \(\frac{a}{b}=-3\)

Khi đó a - b = -3 

a + b = -1.5

=> a = -2,25

=> b = 0,75 

a-b=2(a+b) suy ra a-b=2a+2b suy ra -3b=a

a:b=-3b:b=-3 mà a - b = 2 . ( a + b ) = a : b

suy ra a-b=-3,2(a+b)=-3

suyra a-b=-3,a+b=-3/2

suy ra a-b+a+b=-3+-3/2

suy ra 2a=-9/2

suy ra a=-9/4

thay a=-9/4 vào a-b=-3 suy ra -9/4-b=-3 suy ra b=3/4

vậy....

giỏi lắm đấy các bạn.

A=5+5^3+5^5+5^7+....+5^101

10A=5^3+5^5+5^7+....+5^103

10A-A=(5^3+5^5+5^7+....+5^103)-(5+5^3+5^5+5^7+....+5^101)

9A=5^103-5

A=(5^103-5):9

xy/x+y=12/7 suy ra x+y/xy=7/12 suy ra 1/x+1/y=7/12

tương tự: 1/y+1/z=-1/6,1/z+1/x=-1/4

ta có: 1/x+1/y+1/y+1/z+1/z+1/x=7/12+-1/6+-1/4

2(1/x+1/y+1/z)=1/6

1/x+1/y+1/z=1/12

mà 1/y+1/z=-1/6

suy ra 1/x+-1/6=1/12

suy ra 1/x=1/4

suy ra x=4

thay  1/x=1/4 vào 1/x+1/y=7/12,1/z+1/x=-1/4

1/4+1/y=7/12 suy ra 1/y=1/3 suy ra y=3

1/z+1/4=-1/4 suy ra 1/z=-1/2 suy ra z=-2

vậy x=4,y=3,z=-2

1. When I last saw him, he _____ in London.

A. has lived             B. is living            C. was living             D. has been living

2. We _______ Dorothy since last Saturday.

A. don't see            B. haven't seen           C. didn't see          D. hadn't seen

chắc vậy bạn ạ 

1. D has been living

2. A do not see

Cách giải chung. Đặt \(\frac{a}{b}=\frac{c}{d}=k\Rightarrow a=bk,c=dk\).

5. \(\frac{5a}{a+b}=\frac{5bk}{bk+b}=\frac{5k}{k+1}\)

\(\frac{5c}{c+d}=\frac{5dk}{dk+d}=\frac{5k}{k+1}\)

Suy ra đpcm.

6. \(\frac{a^2+3ab}{a^2-3b^2}=\frac{\left(bk\right)^2+3bk.b}{\left(bk\right)^2-3b^2}=\frac{k^2+3k}{k^2-3}\)

\(\frac{c^2+3cd}{c^2-3d^2}=\frac{\left(dk\right)^2+3dk.d}{\left(dk\right)^2-3d^2}=\frac{k^2+3k}{k^2-3}\)

Suy ra đpcm. 

7, 8. Bạn làm tương tự. 

15. of

16. at

17. in

18. on

19. about

20. of

21. to - with

22. to - by

23. to

24. of

25. of - on - in - of

ĐIỀN GIỚI TỪ

15. Are you tired of  watching TV?

16. She often brings them to  school.

17. He works in a factory.

18. We always go to Ha Long Bay on vacation.

19. Tell me more about your family.

20. Can you send me a photo of your family?

21. He goes to the city with his wife.

22. It takes 18 hours to get to Hanoi by coach.

23. We write to each other twice a week.

24. He is the president of our club.

25. She spends most of the time on the couch in front of the TV.

* Sai xin lỗi ạ *

10. had lived

11. got - had already arrived

12. see - will have graduated

13. visited - was

14. has been - have not you read

15. is washing - has just repaired

10. Margaret was born in 1950. By last year, she (live) ____had live______on this earth for 55 years.

11. The traffic was very heavy. By the time I (get) ____got______to Mary's party, everyone already (arrive) _had already arrived_

12. I will graduate in June. I (see) __see_ you in July. By the time I (see) ___see__ you, I (graduate) _will have graduated_

13. I (visit) ____visited______ my uncle's home regularly when I (be) ______was____ a child.

14. That book (be) ___has been_______ on the table for weeks. You (not read) __have not you read__ it yet?

15. David (wash) ____washing___ his hands. He just (repair) _____has just repaired_____ the TV set.

11.have already begun

12.think - will do

13.is - isn't

14.pass - will start

15.will go

16.can see - have become

11 have already begun 

15 will go 

16 can see / have become

nha bạn mấy câu còn lại mình không biết