I don't know how to play the game.

This department is to the left of the post office.

Phương án 1, nhưng phải sửa là with a ball under the table.

I don't know how to play the game.

This department is to the left of the post office.

Đặt A = \(1+\frac{1}{3}+\frac{1}{6}+\frac{1}{10}+...+\frac{1}{45}\)

\(\frac{1}{2}A=\frac{1}{2}+\frac{1}{6}+\frac{1}{20}+...+\frac{1}{90}\)

\(=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{9.10}\)

\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{9}-\frac{1}{10}\)

\(=1-\frac{1}{10}\)

\(A=\frac{9}{10}.2\)

\(=\frac{9}{5}\)

Đặt \(A=1+\frac{1}{3}+\frac{1}{6}+\frac{1}{10}+...+\frac{1}{45}\)

=> \(\frac{1}{2}A=\left(1+\frac{1}{3}+\frac{1}{6}+\frac{1}{10}+....+\frac{1}{45}\right).\frac{1}{2}\)

              \(=\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+...+\frac{1}{90}\)

             \(=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+....+\frac{1}{9.10}\)

              = \(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+.....+\frac{1}{9}-\frac{1}{10}\)

           = \(1-\frac{1}{10}=\frac{9}{10}\)

=> A= 9/10:1/2=9/5

Ta có \(\frac{1}{3}+\frac{1}{6}+\frac{1}{10}+...+\frac{1}{45}\Rightarrow\frac{1}{2}\left(\frac{1}{3}+\frac{1}{6}+\frac{1}{10}+...+\frac{1}{45}\right)=\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+...+\frac{1}{90}\)
                                                               \(=\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{9.10}\)
                                                                 \(=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+...+\frac{1}{9}-\frac{1}{10}\)
                                                             \(=\frac{1}{2}-\frac{1}{10}=\frac{4}{10}=\frac{2}{5}\Rightarrow\frac{1}{3}+\frac{1}{6}+\frac{1}{10}+...+\frac{1}{45}=\frac{2}{5}:\frac{1}{2}=\frac{4}{5}\)
Vậy \(1+\frac{1}{3}+\frac{1}{6}+\frac{1}{10}+...+\frac{1}{45}=1+\frac{4}{5}=\frac{9}{5}\)

bài ảo

Oxit là hợp chất gồm 2 nguyên tố hóa học trong đó có một nguyên tố là oxy

Chắc là Oxit không ăn mòn á , bạn có thể nghiên cứu trên mạng về õit

Oxit axit là oxit của Phi kim .

Phi kim : ví dụ : S(Lưu huỳnh) ,N(Nitơ) ,C(Cacbon) ,P(Photpho) ...

Các loại oxit axit : ví dụ : \(SO_2,SO_3,NO,CO,CO_2,P_2O_5\)

I. Choose the best answer A, B, C or D to complete the following sentences.

1. The new apartment has a lot of modern things. It is well-__________

A. empty                    B. beautiful               C. furnished                          D. nervous

2. Which is the national anthem of Vietnam?

            A. Tieu Doan 307     B. Chien si Viet Nam C. TienQuanCa                   D. Tienve Ha Noi

3. Can you see any people in the house ? - __________

           A. only a little             B. only few                C. only a few                                   D. only little

4. __________ she plays the piano!

A. How well              B. How good              C. What well                        D. What good

5. After break, everyone goes __________ and classes start again.

           A. indoor                     B. indoors                C. outdoor                                          D. outdoors

6. He learns _______ the guitars everyday.

             A. play                      B. to play                  C. playing                               D. plays

7. __________ my students are familiar with this kind of school activities.

             A. Most                     B. Most of                  C. A few                                D. Few

8.  She looks like you. She __________ your sister.

             A. must be                B. perhaps                  C. must                                  D. may

9. What is the name of the girl __________ to Mrs. Lien?

A. talk                                    B. talks                       C. talking                               D. is talking

10. Would you __________ show me a nice picture, Ann?

A. like                                    B. mind                      C. to                                      D. please

11. Janna can’t play the piano, and Carol can’t play it __________.

            A. either                     B. too                          C. neither                               D. as well

12. Saxophones are used mainly for __________ jazz music.

            A. composing            B. taking                    C. playing                              D. writing

13. I went to the 3D __________ last weekend. The paintings were excellent!

            A. movie theater       B. concert hall          C. opera house                      D. art museum

14. __________ I’m not a fan of country music, I thoroughly enjoyed his lively performance.

            A. Although               B. Because                 C. As                                       D. However

15. __________ does a water puppet show take place? ~ In a pool.

            A. What                      B. When                     C.How                                    D. Where

1C ;))

2C