\(\dfrac{1}{1\cdot2}+\dfrac{1}{2\cdot3}+...+\dfrac{1}{9\cdot10}\)

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

\(=1-\left(\dfrac{1}{2}-\dfrac{1}{2}\right)-\left(\dfrac{1}{3}-\dfrac{1}{3}\right)-...-\left(\dfrac{1}{9}-\dfrac{1}{9}\right)-\dfrac{1}{10}\)

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

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

\(=\dfrac{9}{10}\)

Lời giải:

$A=16-18+20-22+24-26+...+64-66+68$

$=\underbrace{(16+20+24+...+64+68)}_{M}-\underbrace{(18+22+26+...+66)}_{N}$

M có số số hạng là:

$(68-16):4+1=14$

Giá trị $M$ là: $(68+16)\times 14:2=588$

N có số số hạng là:

$(66-18):4+1=13$

Giá trị N là: $(66+18)\times 13:2=546$

$A=M-N=588-546=42$

chỉ vui vẻ th=)

\(m_C=\dfrac{46.52,17}{100}=24g\\ m_H=\dfrac{46.13,05}{100}=6g\\ m_O=\dfrac{46.34,78}{100}=16g\\ n_C=\dfrac{24}{12}=2mol\\ n_H=\dfrac{6}{1}=6mol\\ n_O=\dfrac{16}{16}=1mol\\ \Rightarrow CTHH:C_2H_6O\)

Kết quả là 61/18 hay 3 , 38 nha !

61/18 hoặc 3 , 38 

a - b + 31344

Thay a = 23 và b= 32490  vào biểu thức a - b + 313 44 ta có:

   a - b + 31344 = 23 - 32490 + 31344

                         = (23 + 31344) - 32490

                        = 31367 - 32490

                       =  - 1123

-1123

giúp mình nhanh nhé mai mình nộp rồi!!!

Exercise 9: Write question for the underlined words:

1. I often go shopping with my friends in my free time.

sdsg=> ............What do you often do in your free time?..............................................................................

2. Lan goes to the library twice a week.

sdsg=> ...How often does Lan go to the library?.......................................................................................

3. The go camping by bus.

sdsg=> ..How do they go camping?........................................................................................

4. Yes. She sometimes talks to her friends on the phone.

sdsg=> .......................Does she sometimes talks to her friends on the phone?..................................................................

5. She usually watches TV in the evening.

sdsg=> .........What does she usually do in the evening?.................................................................................

6. They have English lessons three times a week.

sdsg=> ......How often do they have English lessons?....................................................................................

7. My mother gets up at 6 a.m every day. 

sdsg=> ..What time does your mother get up every  day?........................................................................................

8. Nam has Maths on Tuesday and Thursday.

sdsg=>.....When does Nam have Maths?.......................................................................................

9. My parents live in a small house in Da Nang.

sdsg=> ..Where do your parents live?........................................................................................

10. Her address is 102, Nguyen Trai Street, Ha Noi.

sdsg=> ..What is her address?........................................................................................

Exercise 10: Read and answer the questions.5

          My name is Huong. I usually get up at six o’clock in the morning. I domorning exercise with my father before breakfast. I have a big breakfast withmy family at 7 o’clock. After breakfast, my sister and I walk to school. Afterschool, I often play badminton with my friends. I play badminton three times aweek. Then I go home and help my mother cook dinner. I have dinner with myfamily at 6.30 p.m. Next, I usually do my homework. After finishing myhomework, I sometimes listen to music of surf the Internet to look for someinformation. I usually go to bed at 9 o’clock.

1. What’s her name?

sdsg=> ..........Her name is Huong................................................................................

2. What time does she usually get up in the morning?

sdsg=> ......She usually gets up at 6 o'clock in the morning....................................................................................

3. What does she do before breakfast?

sdsg=> ......She does morning exercise with her father before breakfast....................................................................................

4. Who does she go to school with?

sdsg=> .........She goes to school with her sister.................................................................................

5. Does she often play soccer after school?

sdsg=> .............No, she doesn't.............................................................................

6. How often does she play badminton?

sdsg=> .............She plays badminton three times a week.............................................................................

7. What does she usually do after dinner?

sdsg=> .She usually does her homework after dinner.........................................................................................

8. Does she sometimes surf the Internet in the evening?

sdsg=> .Yes, she does.........................................................................................

9. What time does she go to bed?

sdsg=> .................She goes to bed at 9 o'clock.........................................................................