Gọi giao điểm của hai đường chéo là O

Độ dài cạnh OA là: \(OA=\dfrac{1}{2}\cdot AC=\dfrac{1}{2}\cdot10=5\left(cm\right)\)

Ta có \(\Delta AOD\) vuông tại A nên ta có:

\(AD^2=OA^2+OD^2\)

\(\Rightarrow OD=\sqrt{AD^2-OA^2}=\sqrt{13^2-5^2}=12\left(cm\right)\)

Mà: \(OD=\dfrac{1}{2}BD\Rightarrow BD=2\cdot OD=2\cdot12=24\left(cm\right)\)

Vậy chọn đáp án A

Chọn A

1, How long

2, Which

3, Who

4, What

5, How many

1.___how long___ have you give up smoking? For about two years.

2.___which___ place is more densely populated-Ha Noi or Ho Chi Minh City?

3.__who__ broke Jane's favorite vase? Her brother did it.

4.__what__ does Mr.Brown do? He's a researcher

5.__how many__ ethnic groups are there in your country?

1. My brother hasn't seen his best friend for nearly five years.

=> It's five years since my brother last met his best friend.

2. When did you start learning English?

=> How long have you studied English?

3. Minh began to collect stamps in 2000.

=> Minh has begun to collect stamps since 2000.

Hok tốt

1. My brother hasn't seen his best friend for nearly five years

=> It's is the first time my brother saw him after nearly five years

2. When did you start learning English?

=> How long have you learnt English ?

3. Minh began to collect stamps in 2000

=> Minh has collected stamp from 2000 .

1. My brother hasn't seen his best friend for nearly five years

=> It's is the first time my brother saw him after nearly five years

2. When did you start learning English?

=> How long have you learnt English ?

3. Minh began to collect stamps in 2000

=> Minh has collected stamp from 2000 .

1.Where is your younger brother?”, Minh asked me.

➔ Minh asked me where my younger brother was.

2. “Say goodbye to my grandparents”, they said.

➔ They asked me to say goodbye to their grandparents.

3. “ Did Mr Bean send the letter to you?”, Mai asked him.

➔ Mai asked him if Mr Bean had sent letter to him.

4. “Where is the best place to eat pizza?”, Oanh asked.

➔ Oanh asked where the best place to eat pizza was.

5. “What do you want for dinner tonight, Ly?”, he asked.

➔ He asked Ly what she wanted for dinner that night.

6. Jun asked me, “Did you go to work yesterday?”

➔ Jun asked me if I had gone to work the day before.

7. My sister asked him, “How long have you learnt English?”

➔ My sister asked him how long he had learnt English.

8. She wanted to know, “Are you a teacher?”

➔ She wanted to know if I was a teacher.

9. My grandfather asked her, “Does you enjoy your job?”

➔ My grandfather asked her if she enjoyed her job.

10 .He asked, “When did you buy it?”

➔ He asked me when I had bought it.