Vì \(\Delta\)ABC cân tại A \(\rightarrow AB=AC;\widehat{ABC}=\widehat{ACB}\)

Xét \(\Delta AHB\) và \(\Delta AKC\) có:

\(\widehat{A}\) chung

AH=AK (gt)

AB=AC

\(\rightarrow\Delta AHB=\Delta AKC\left(cgc\right)\)

\(\rightarrow\widehat{ABH}=\widehat{ACK}\) (2 góc tương ứng)

Ta có: \(\widehat{ABC}=\widehat{OBC}+\widehat{ABH}\); \(\widehat{ACB}=\widehat{OCB}+\widehat{ACK}\)

\(\widehat{ABC}+\widehat{ACB}\rightarrow\widehat{OBC}+\widehat{ABH}=\widehat{OCB}+\widehat{ACK}\)

mà \(\widehat{ABH}=\widehat{ACK}\)

\(\rightarrow\widehat{OBC}=\widehat{OCB}\)

\(\Rightarrow\Delta OBC\) cân tại O (đpcm)

1. I am doing my homework

2. Bella is talking on the phone

3. We are listening to CDs now

4. The children are riding to the park

5. It is raining very hard now

6. She is baking a chocolate cake

7. You are dancing very beautifully

8.Mark and his friends are having lunch

1. am looking

2.Do you know

3. Are they doing

4. often play

5. don't like

6. watches

7. are doing

8.doesn't do

9. is playing

1. aren't helping

2. isn't staying

3. am not traveling

4. aren't studying

5. aren't playing

6. isn't watching

7. aren't talking

8. isn't shining

1.pollutes

2.rubbish

3.amount

4.because

5.burning

6.should

7.who

8.and

9.of

10.green

Nói chung là thi xong văn thì nghe tương lai nó mờ mờ hơn 1 chút r

Văn thì bài nào cũng hay, mỗi tội mk ko đủ trình để cảm nhận nó. Nghĩ nó chán

Ta có: \(M=3+3^2+3^3+...+3^{20}\)

\(M=\left(3+3^2\right)+\left(3^3+3^4\right)+...\left(3^{19}+3^{20}\right)\)

\(M=\left(3+3^2\right)\cdot1+\left(3+3^2\right)\cdot3^2+...+\left(3+3^2\right)\cdot3^{18}\)

\(M=\left(3+3^2\right)\left(1+3^2+...+3^{18}\right)\)

\(M=12\left(1+3^2+...+3^{18}\right)\)

\(\rightarrow M⋮12\)

Mà 12\(⋮\) 3 và 12\(⋮\)4

\(\Rightarrow\) M chia hết cho cả 3 và 4

Một môn cuối cùng ( t sợ ngang ngửa Văn)