vì sao các phân tử nước có kĩ năng liên kết với nhau ? Đặc tính này có ý nghĩa dưới sự sống như thế nào

cho tam giác ABC có D,E,F lần lượt là trung điểm của BC , CA, AB. Gọi M là trung điểm của AD . Chứng minh

a, \(2\overrightarrow{AM}+\overrightarrow{MB}+\overrightarrow{MC}=\overrightarrow{0}\)

b, \(2\overrightarrow{OA}+\overrightarrow{OB}+\overrightarrow{OC}=4\overrightarrow{OM}\)( O tùy ý)

c, \(\overrightarrow{AD}+\overrightarrow{BE}+\overrightarrow{CF}=\overrightarrow{0}\)

cho tam giác ABC có trọng tâm G. Gội H là điểm đối xứng của B qua G

a, chứng minh \(\overrightarrow{AH}=\frac{2}{3}\overrightarrow{AC}-\frac{1}{3}\overrightarrow{AB}\) và \(\overrightarrow{CH}=-\frac{1}{3}\left(\overrightarrow{AB}+\overrightarrow{AC}\right)\)

b, gọi M là trung điểm của BC. CHứng minh \(\overrightarrow{MH}=\frac{1}{6}\overrightarrow{AC}-\frac{5}{6}\overrightarrow{AB}\)

1, I' d like to stay at home at home instead of............to the cinema

a.go b.going c.went d.to go

2.i would rather............unemployed than.............in that factory

a.to be/to work b. be/work c. being/work d.being/working

3. mary was so angry that she..........to see the manager

a.demanded b. hoped c. risked d. stopped

4. i don,t think that red dress...........her

a. suits b.matches c.cheers d.agrees

5. the weather is............awful that i don,t like going out this evenning

a. such b. so c. too d. very

6. let your name be written.............the sheet of raper

a.by b. on c. with d. of

7. my father had a worker............a fence round our garden

a. make b. making c. to make d. made

8. i think she is accustomed to.............english at a private school

a. taught b teach c. teaching d. be taught

9. the three fiends all...for the same job

a. requested b. applied c. intended d. referred

10. the children could hardly..........to leave their pets behind

a. forget b. regret c reding d. have read

11. this book was written by a well known writer. It is worth.......

a. read b. to read c. reading d. have read

12 she remembered the correct address only.............she had posted letter

a. since b. afferwards c. following d. after

13. you don't mind looking.... ..the baby , do you?

a. after b for c. at d. in

14. his parents never allowed him.......

a. a smoking b. smoking c. to smoke d. smoke

15. do you........to know when this castle was built?

a. ask b. happen c. imagine d. like

1, I wish (see)...........the manager

2, I'd like him ( start).........(take)..............more English lessons

3, I have no intention of (go)...........to the play

4, I hate (get)............up early and (dress)......in the park

5, She was the last stundent (leave) ............the room

6, I arranged (meet) ....them later

7, Our teacher has promised (help)............us (prepare)..........for the exam

8, The movie is worth (see)........

9, I'd like (have).............an opportunity of (meet)...............again

10, I had to ask the boys ( stop)...........(play)............

11, he is thinking of (quit)..........his job

12, please let me (know).....your decision

cho hình thoi ABCD cạnh a và góc BCD = 60 độ. Gọi O là tâm của hình thoi. Tính \(\left|\overrightarrow{AB}+\overrightarrow{AD}\right|\), \(\left|\overrightarrow{OB}-\overrightarrow{DC}\right|\)

cho hình vuông ABCD có tâm O và cạnh a. M là điểm bất kỳ

a, Tính |\(\overrightarrow{AB}+\overrightarrow{OD}\)| , \(\left|\overrightarrow{AB}-\overrightarrow{OC}+\overrightarrow{OD}\right|\)

b, Tính độ dài vecto \(\overrightarrow{MA}-\overrightarrow{MB}-\overrightarrow{MC}+\overrightarrow{MD}\)

cho tam giác ABC đều cạnh a. Tính độ dài của các vecto sau \(\overrightarrow{AB}-\overrightarrow{AC},\overrightarrow{AB}+\overrightarrow{AC}\)