Ta có \(\frac{bz-cy}{a}=\frac{cx-az}{b}=\frac{ay-bx}{c}\)

\(\Leftrightarrow\frac{abz-acy}{a^2}=\frac{bcx-abz}{b^2}=\frac{acy-bcx}{c^2}\)

Áp dụng t/c dãy tỉ số bằng nhau : \(\frac{abz-acy}{a^2}=\frac{bcx-abz}{b^2}=\frac{acy-bcx}{c^2}=\frac{abz-acy+bcx-abz+acy-bcx}{a^2+b^2+c^2}=0\)

Từ đó suy ra đpcm

a/ Hai nguyên tử Oxi : 2O

b/ Ba phân tử Canxi Hidroxit : 3Ca(OH)₂

c/ Bảy phân tử Amoniac : 7NH₃

The boys repeatedly asked for permission to go out with his friends.

People say that he is a troubled child in his family

a. I(visit) will visit Hue this summer vacation.

b. Viet and Long often(play) play video games.

c.How does your father(go) go to work every day?

   - He(go)goes to work by motobike.

d. She can (ride) ride a bike but she can't drive(drive).

e. My sister(have) has a nice cat.She(not have) doesn't have a bird.

f.One with one(be) is two.

g.The earth(move) moves around the sun.

1. Lan did not know the world. Hoa didn’t, too

2. The red pencil is yours , the green one is mine

3. My little sister never stays up late. She always goes to bed early.  

4. There are some trees in front of my yard. 

5. ’’Beside’’ has similar meaning to ’’next to’’. 

6. - I have a headache. - You should take some aspirins. 

7. Kent sits between Hilary and John. 

i/ not eat / any thing/ breakfast

=> I haven't eaten anything for breakfast

you/ ever/ eat / cheese?

=> Have you ever eaten cheese?

your mom/ ever/ watch/this film?

=> Has your mom ever watched this film?

we /already /visit / ha long bay/twice

=> We have already visited Ha Long bay twice

i / not receive/any letters/ from maryam/ recently

=> I haven't received any letters from Maryam recently

my brother/ never/ be / ha noi

=> My brother has never been to Hanoi

hoang/ just / buy/ a new bike

=> Hoang has just bought a new bike

this/ first time/ i meet/ pen pal

=> This is the first time I've met pen pal

you/ ever/ speak/ a famous person?

=> Have you ever spoken with a famous person?

mrs. ha / never/ travel/ by plane

=> Mrs.Ha has never traveled by plane

Mỗi lít dầu cân nặng:

29,25 : 65 = 0,45 (kg)

Khối lượng dầu chứa trong can:

12 – 1,2 = 10,8 (kg)

Số lít dầu chứa trong can:

10,8 : 0,45 = 24 (lít)

Đáp số:  24 lít

Áp dụng định lí Cosin : 

\(BC^2=AB^2+AC^2-2AB.AC.cosA\)

a ) Lan is learning to dance .

She is going to be a dancer

b ) Hoa loves children .

She is going to be a teacher

c ) Tom and John are learning to fly

They are going to be pilots

d ) Nam is studying law .

He is going to be a lawer

e ) Bob's at drama school 

He is going to be an actor

f ) Peter cooks very well .

He is going to be a chef

g ) Loan's good at languages .

She is going to be an interviewer

h ) Linda can run very fast .

She is going to be an athlete

i ) Thu is very good  at singing

She is going to be a singer