dễ 

tick mình làm cho câu a

Hello everybody! My name is ........Iam ........ years old. Today, I will tell for you my hobby!

My hobby is a interesting hobby. It is listening to music. I started it when I was 7. I have listened to music in Youtube and radio. I like every English songs so I have many English songs DVD. I often do it in my free time and go to the shops to buy some music DVD. My idols is Justin Bieber. He is " The king's pop".

I think my hobby is a good hobby because it held me more relax and advanced foreign language skills. I hope may be you can do my hobby and you will love it!

a, Vì OA < OB (2<4) nên điểm O nằm giữa hai điểm B và Ở (1)

b, Từ (1) ta co:

OA + AB = OB

Suy ra 2 + AB = 4

Suy ra AB = 4-2= 2

Vậy AB = OA (2)

c, Từ (1) và (2) ta có điểm A chính là trung điểm của OB