Cho Tam giác ABC có 3 góc nhọn ( AB<AC) nội tiếp (O;R) hai đừng AD và BE cắt tại H . Gọi M,N lần lượt là giao điểm của (O) với các tia BE và AD ( M khác B , N khác A) a/ tứ giác ABDE nội tiếp b/ chứng minh DE song song MN ( cách đồng vị )
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15 tháng 6 2022
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
A
2 tháng 7 2019
Gọi ( O;R ) , ( I ;r ) lần lượt là các đường tròn ngoại tiếp tam giác ABC, DEF
Tam giác ABC ~ Tam giác DEF ( vì \(\widehat{ABC}=\widehat{DEF};\widehat{BAC}=\widehat{EDF}\)) \(\Rightarrow\widehat{ABC}=\widehat{DEF}\)
\(\widehat{ACB},\widehat{DEF}\)nhọn nên \(\widehat{ACB}=\frac{1}{2}\widehat{AOB};\widehat{DEF}=\frac{1}{2}\widehat{DIE}\)( hệ quả góc nội tiếp )
\(\Rightarrow\widehat{AOB}=\widehat{DIE}\)
\(OA=OB\left(=R\right)\Rightarrow\Delta OAB\)cân tại O
\(ID=IE\left(=r\right)\Rightarrow\Delta IDE\)cân tại I
Do đó Tam giác OAB ~ Tam giác IDE \(\Rightarrow\frac{OA}{ID}=\frac{AB}{DE}\Rightarrow\frac{R}{r}=\frac{3DE}{DE}\)
\(\Rightarrow R=3r\) ( đpcm)
5 tháng 7 2019
Gọi ( O; R ), ( I; R ) lần lượt là các đường tròn ngoại tiếp tam giác ABC, DEF
Tam giác ABC ~ Tam giác DEF ( vì \(\widehat{ABC}=\widehat{DEF;}\widehat{BAC}=\widehat{EDF}\) ) \(\Rightarrow\widehat{ABC}=\widehat{DEF}\)
\(\widehat{ABC}=\widehat{DEF}\)nhọn nên \(\widehat{ACB}=\frac{1}{2}\widehat{AOB};\widehat{DEF}=\frac{1}{2}\widehat{DIE}\)(hệ quả góc nội tiếp )
\(\Rightarrow\widehat{AOB}=\widehat{DIE}\)
\(OA=OA\left(=R\right)\Rightarrow\Delta OAB\)cân tại O
Do đó Tam giác OAB ~ Tam giác IDE\(\Rightarrow\frac{OA}{ID}=\frac{AB}{DE}\Rightarrow\frac{R}{r}=\frac{3DE}{DE}\)
\(\Rightarrow R=3r\left(đpcm\right)\)
Rất vui vì giúp đc bạn <3
a: Xét tứ giác ABDE có \(\hat{AEB}=\hat{ADB}=90^0\)
nên ABDE là tứ giác nội tiếp
b: Xét (O) có
\(\hat{BMN};\hat{BAN}\) là các góc nội tiếp chắn cung BN
=>\(\hat{BMN}=\hat{BAN}\)
mà \(\hat{BAN}=\hat{BAD}=\hat{BED}\) (ABDE nội tiếp)
nên \(\hat{BED}=\hat{BMN}\)
mà hai góc này là hai góc ở vị trí đồng vị
nên ED//MN