Đặt A = \(\frac{1}{4.9}+\frac{1}{9.14}+\frac{1}{14.19}+...+\frac{1}{44.49}\)

\(\Rightarrow\) A = \(\frac{1}{5}\left(\frac{5}{4.9}+\frac{5}{9.14}+\frac{5}{14.19}+...+\frac{1}{44.49}\right)\)

\(\Rightarrow\) A = \(\frac{1}{5}.\left(\frac{1}{4}-\frac{1}{9}+\frac{1}{9}-\frac{1}{14}+\frac{1}{14}-\frac{1}{19}+...+\frac{1}{44}-\frac{1}{49}\right)\)

\(\Rightarrow\) A = \(\frac{1}{5}\left(\frac{1}{4}-\frac{1}{49}\right)\)

\(\Rightarrow\)A = \(\frac{1}{5}.\frac{45}{196}=\frac{9}{196}\)

Đặt B = \(\frac{1-3-5-7-9-...-49}{89}\)

\(\Rightarrow\)B = \(\frac{1-\left(3+5+7+9+...+49\right)}{89}\)

\(\Rightarrow\)B = \(\frac{1-624}{89}=-7\)

Vậy M =\(\frac{9}{196}.-7=-\frac{9}{28}\)

Ta có : \(\frac{bz-cy}{a}=\frac{cx-az}{b}=\frac{ay-bx}{c}\)= \(\frac{bza-cya}{a^2}=\frac{cxb-âzb}{b^2}=\frac{ayc-bxc}{c^2}\)

= \(\frac{bza-cya+cxb-azb+ayc-bxc}{a^2+b^2+c^2}\)\(=\frac{0}{a^2+b^2+c^2}=0\)

Suy ra : bz - cy = 0 \(\Rightarrow\) bz= cy \(\Rightarrow\) \(\frac{z}{c}=\frac{y}{b}\) (1)

cx - az = 0 \(\Rightarrow\) cx = az \(\Rightarrow\) \(\frac{x}{a}=\frac{z}{c}\) (2)

ay - bx = 0 \(\Rightarrow\) ay = bx \(\Rightarrow\)\(\frac{y}{b}=\frac{x}{a}\) (3)

Từ (1) , (2) và ( 3) \(\Rightarrow\)\(\frac{x}{a}=\frac{y}{b}=\frac{z}{c}\) (điều phải chứng minh )

làm hết thì mk ko làm nổi đâu, bạn chọn câu nào khó nhất rồi mk làm cho

1. You had better not eat too much candies because they are not good for your teeth

2. I have anamerican friend .

3. Last week they moved to the new house in the city .

4.Last year my brother worked as an engineer in a printing factory

5.What is the width of the West lake ?

6.This morning I ate bread and drank milk for breakfast

7. Walking is a funny , easy and inexpensive activity .

8.Most of the world surface is water

9.Would you like some coffee?

10. Video games are good fun, but players must be careful.

7. I am pleased that you and your family are well

8. Here is a photo of my family and let me tell you about us

9. My father is a mechanic and he works in a factory in the suburb

10. He works five days a week from Monday to Friday

11.He usually goes to work by motorbike in the morning so he is not free every morning

12. In free time, my father often plays badminton with his friends

13. My mother is a teacher . She teaches Math at a school near my house

14. My brother is 17 years old and he is in grade 11

15. He lovescollecting stamps . He has hundreds of stamps in his collection.

16. Please write to me soon and tell me about your family.

Snow White was the daughter of a beautiful queen, who died when the girl (be)1--> was young. Her father (marry)2--> married again, but the girl’s stepmother was very jealous of her because she was so beautiful. The evil queen (order)3--> ordered a hunter to kill Snow White but he couldn’t do it because she was so lovely. He (chase)4--> chased her away instead, and she (take)5--> took refuge with seven dwarfs in their house in the forest. She (live)6--> lived with the dwarfs and took care of them and they (love)7--> loved her dearly. Then one day the talking mirror (tell)8--> told the evil queen that Snow White was still alive. She (change)9--> changed herself into a witch and (make)10--> made a poisoned apple. She (go)11--> went to the dwarfs’ house disguised as an old woman and tempted Snow White to eat the poisoned apple, which (put)12--> put her into an everlasting sleep. Finally, a prince (find)13--> found her in the glass coffin where the dwarfs had put her and woke her up with a kiss. Snow White and the prince (be)14--> were married and lived happily ever after.

Khí hậu Bắc Mỹ có sự phân hoá rất đa dạng:
- Từ Bắc xuống Nam , có 3 vành đai khí hậu: hàn đới, ôn đới, nhiệt đới.
Đây là sự phân hoá khí hậu theo vỹ độ ( quy luật địa đới, độ vỹ càng cao góc tới càng nhỏ, lượng bức xạ mặt trời càng ít…) Do địa hình Bắc Mĩ nằm từ vòng cực Bắc đến 15\(^0\) B .
- Tuy nhiên, khi đi từ Bắc xuống Nam, mỗi đới khí hậu lại có sự phân hoá theo chiều từ tây sang đông .Do Hệ thống Cooc-đi-e đồ sộ chạy từ B-N ngăn cản khối khí từ đại dương vào đất liền .
- Ngoài ra, khí hậu của Bắc Mỹ cũng phân hóa theo độ cao :
+ Chân núi có khí hậu cận nhiệt hay ôn đới tuỳ thuộc vào vị trí.
+ Lên cao thời tiết lạnh dần , nhiều đỉnh cao có băng tuyết vĩnh cữu.