Vẽ \(CI\perp AB\), \(ED\perp CA\).
\(\Delta ABC\) cân tại C có CI là đường cao
\(\Rightarrow\) CI là đường trung tuyến, cũng là đường phân giác.

Ta có: CI là đường phân giác của \(\Delta ABC\)
\(\Rightarrow\) \(\widehat{ACI} = \widehat{BCI}\) \(=\dfrac{\widehat{C}}{2}\)
\(\Rightarrow\) \(\widehat{ACI} = \widehat{BCI}\) \(=\dfrac{120^0}{2}=60^0\)
Ta có: CI là đường trung tuyến của \(\Delta ABC\)
\(\Rightarrow IA=IB=\dfrac{1}{2}AB\)
\(\Rightarrow IA=IB=\dfrac{3a}{2}=1,5a\)

Ta có: \(\widehat{A} = \widehat{B}\) \(=\dfrac{180^0-\widehat{C}}{2}\) (\(\Delta ABC\) cân tại C)
\(\Rightarrow\) \(\widehat{A} = \widehat{B}\) \(=\dfrac{180^0-120^0}{2}=30^0\)
\(\Delta EAD\) vuông tại E có \(\widehat{A}=30^0\)
\(\Rightarrow ED=\dfrac{1}{2}AD=0,5a\)
Có \(DI=AI-AD\)
\(\Rightarrow DI=1,5a-a\)
\(\Rightarrow DI=0,5a\)

Hai tam giác vuông CDE và CDI có:
CD là cạnh chung
ED = DI (= 0,5a)
\(\Rightarrow\Delta CDE=\Delta CDI\) (cạnh huyền - cạnh góc vuông)
\(\Rightarrow\widehat{C_1}=\widehat{C_2}\) (hai góc tương ứng)
\(\Rightarrow\widehat{C_1}=\widehat{C_2}=\dfrac{\widehat{ACI}}{2}=\dfrac{60^0}{2}=30^0\)

\(\Delta CDE\) có \(\widehat{C_1}=30^0\Rightarrow ED=\dfrac{1}{2}CD\)
Có: \(ED=\dfrac{1}{2}AD\), \(ED=\dfrac{1}{2}CD\)
\(\Rightarrow CD=CA\)
mà DA = a \(\Rightarrow\) CD = a
Vậy CD = a.

3. Come in and have a seat, please.

2. Thanks. oh, what a nice living room!

4. It's very kind of you to say so. How about a drink?

6. Yes, I'd love some. Where's your room?

1. It's over there. Let's see my room.

5. Oh, it's so bright. What nice colors! Pink is my favorite color.

10. Would you like to see the rest of the house?

12. Of course.

7. This is the bathroom and that's the kitchen.

9. What a modern kitchen! It has everything.

11. Yes, it has a new refrigerator and a new dishwasher.

8. What a lovely house!

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Chuyển câu chủ động sang câu bị động :

1, The examiner will read the passage three times.
=> The passage will be read by the examiner three times.

2, They will pick me up at my house at 7 o clock tomorrow morning.
=> I will be picked up at my house at 7 o'clock tomorrow morning.

3, The teacher won't corrert exercise tomorrow.
=> Exercise won't be corrected by the teacher tomorrow.

4, My cousin will meet you at the station.
=> You will be met by my cousin at the station.

5, I will not forgive her for the serious mistake she has made.
=> She will not be forgiven by me for the serious mistake she has made.