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8 tháng 1 2024
Gọi D là giao điểm BM và CN.
Trên cạnh BC lấy điểm E sao cho \(BE=BN\)
Khi đó \(CE=BC-BE=BN+CM-BE=CM\)
Xét hai tam giác BDE và BDN có:
\(\left\{{}\begin{matrix}BE=BN\\\widehat{DBE}=\widehat{DBN}\left(\text{BM là phân giác}\right)\\BD\text{ chung}\end{matrix}\right.\) \(\Rightarrow\Delta BDE=\Delta BDN\left(c.g.c\right)\)
\(\Rightarrow\widehat{BDE}=\widehat{BDN}\)
Hoàn toàn tương tự, ta cũng có \(\Delta CDE=\Delta CDM\left(c.g.c\right)\Rightarrow\widehat{CDE}=\widehat{CDM}\)
Mà \(\widehat{BDN}=\widehat{CDM}\) (đối đỉnh) \(\Rightarrow\widehat{BDN}=\widehat{BDE}=\widehat{CDM}=\widehat{CDE}\)
Mà \(\widehat{BDE}+\widehat{CDE}+\widehat{CDM}=180^0\)
\(\Rightarrow3\widehat{BDE}=180^0\Rightarrow\widehat{BDE}=60^0\)
\(\Rightarrow\widehat{CDE}=60^0\)
\(\Rightarrow\widehat{BDC}=\widehat{BDE}+\widehat{CDE}=120^0\)
Theo tính chất tổng 3 góc tổng tam giác:
\(\widehat{BDC}+\widehat{DBC}+\widehat{DCB}=180^0\)
\(\Rightarrow120^0+\dfrac{1}{2}\widehat{B}+\dfrac{1}{2}\widehat{C}=180^0\)
\(\Rightarrow\widehat{B}+\widehat{C}=120^0\)
Do tổng 3 góc trong tam giác ABC bằng 180 độ
\(\Rightarrow\widehat{A}+\widehat{B}+\widehat{C}=180^0\)
\(\Rightarrow\widehat{A}+120^0=180^0\)
\(\Rightarrow A=60^0\)
15 tháng 12 2021
a. Chu vi là \(\left(12+5\right).2=34\left(m\right)\)
Diện tích là \(12.5=60\left(m^2\right)=600000\left(cm^2\right)\)
b. Cần lát \(600000:\left(40.40\right)=375\) viên gạch
17 tháng 3 2022
TL:
Sai nhé bạn
Bạn k cho mik cái đi nhé
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HT
NT
Ai giúp mình với ạ
0
1. give / things / him / many / the foreman / this morning / to do / How / did ?
How many things did the foreman give him this morning?
2. having / of / the moment / Tran Phu School / check-up / The students / at / medical / are / a /
The students of Tran Phu School are having a check-up medical at teh moment.
3. yesterday / go / a / had / Lan / school / headache / didn't / because / to / she /.
Lan didn't go to yesterday because she had a headache.
4. catch / milions / cold / Every / people / the / of / year / common.
Every year millions of people catch the common cold.
5. last / days / disappears / cold / for / common / few / then / The / a / and /.
The common cold last for a few days and then disappears.
6. prevent / and / well / a cold / You / doing / eating / can / exercise / help / by /.
You can help prevent a cold by eating well and doing exercise.
7. terrible / the dentist / had / visited / a / toothache / she / because / Trang / yesterday/.
Trang visited the dentise yesterday because she had a terrible toothache.
8. breathing / the facemask / from / Wearing / helps me / in / stop / dust /.
Wearing the facemask helps me stop from breathing in dust.
9. absence / ago / my / a / days / father / note / My / two / sick / wrote / for /.
My father wrote a sick note for my absence two days ago.
10. unpleasant / knows / very / Catching / a cure / but / the common cold / no one / is /.
Catching the common cold is very unpleasant buy no one knows a cure.
Chúc bạn học tốt ^^