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3 tháng 5 2019

a) Áp dụng pytago .

b) Xét t/g ABE; tg DBE:

AB = DB ( gt)

g ABE = DBE (suy từ gt)

BE chung

=> tg ABE = tg DBE (c.g.c)

c) Vì tg ABE = tg DBE (câu b)

=> AE = DE

Xét tg AEF ⊥⊥ tại A; tg DEC ⊥⊥ tại D:

AE = DE (c/m trên)

g AEF = g DEC (đối đỉnh)

=> tg AEF = tg DEC (cgv - gn)

=> EF = EC

d) Do tg AEF = tg DEC (câu c)

=> AE = DE

=> E ∈∈ đg trung trực của AD (1)

Lại do AB = BD (gt)

=> B  đg trung trực của AD (2)

Từ (1) và (2) => BE là đg trung trực của AD.

1 tháng 5 2020
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8 tháng 5 2022

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\(\text{a)Xét }\Delta ABC\text{ vuông tại A có:}\)

\(BC^2=AB+AC^2\left(\text{định lí Py ta go}\right)\)

\(\Rightarrow BC^2=5^2+7^2=25+49=74\left(cm\right)\)

\(\Rightarrow BC=\sqrt{74}\left(cm\right)\)

\(\text{b)Xét }\Delta ABE\text{ và }\Delta DBE\text{ có:}\)

\(\widehat{BAE}=\widehat{BDE}=90^0\left(gt\right)\)

\(BE\text{ chung}\)

\(BA=BD\left(gt\right)\)

\(\Rightarrow\Delta ABE=\Delta DBE\left(c-g-c\right)\)

\(\text{c)Xét }\Delta AEF\text{ và }\Delta DEC\text{ có:}\)

\(\widehat{AEF}=\widehat{DEC}\left(\text{đối đỉnh}\right)\)

\(\widehat{FAE}=\widehat{CDE}=90^0\left(gt\right)\)

\(AE=DE\left(\Delta ABE=\Delta DBE\right)\)

\(\Rightarrow\Delta AEF=\Delta DEC\left(g-c-g\right)\)

\(\Rightarrow EF=EC\left(\text{hai cạnh tương ứng}\right)\)

\(\text{d)Gọi O là giao điểm của BE và AD}\)

\(\text{Xét }\Delta ABO\text{ và }\Delta DBO\text{ có:}\)

\(BO\text{ chung}\)

\(BA=BD\left(gt\right)\)

\(\widehat{ABO}=\widehat{DBO}\left(\Delta ABE=\Delta DBE\right)\)

\(\Rightarrow\Delta ABO=\Delta DBO\left(c-g-c\right)\)

\(\Rightarrow\widehat{AOB}=\widehat{DOB}\left(\text{hai góc tương ứng}\right)\)

\(\text{Mà chúng kề bù}\)

\(\Rightarrow\widehat{AOB}=\widehat{DOB}=\dfrac{180^0}{2}=90^0\)

\(\Rightarrow BE\perp AD\)

\(\text{Mà AO=DO}\left(\Delta AOB=\Delta DOB\right)\)

\(\Rightarrow BE\text{ là đường trung trực của đoạn thẳng AD}\)

8 tháng 5 2022

cảm ơn bạn nghe thank you mà làm thế này đúng ko bạn:

a) Vì tam giác BAC vuông tại A

=> AB^2 + AC^2 = BC^2 ( đl pytago )

=> BC^2 = 5^2 + 7^2 = 74

=> BC = căn bậc 2 của 74

b)

Xét tam giác ABE; tam giác DBE có :

AB = DB ( gt)

góc ABE = góc DBE ( gt)

BE chung

=> tam giác ABE = tam giác DBE (c.g.c) - đpcm

c)

Vì tam giác ABE = tam giác DBE (câu b)

=> AE = DE

Xét tg AEF ⊥ tại A; tg DEC ⊥ tại D:

AE = DE (c/m trên)

g AEF = g DEC (đối đỉnh)

=> tg AEF = tg DEC (cgv - gn) - đpcm

=> EF = EC

d)

Do tam giác AEF = tam giác DEC (câu c)

=> AE = DE

=> E ∈ đường trung trực của AD (1)

Lại do AB = BD (gt)

=> B ∈ đường trung trực của AD (2)

Từ (1) và (2) => BE là đường trung trực của AD. - đpcm

3 tháng 5 2020

A B C D F E

a) Vì tam giác BAC vuông tại A 

=> AB^2 + AC^2 = BC^2 ( đl pytago )

=> BC^2 = 5^2 + 7^2 = 74

=> BC = căn bậc 2 của 74

b) 

 Xét tam giác ABE; tam giác DBE có :

AB = DB ( gt)

góc ABE = góc DBE ( gt)

BE chung

=> tam giác ABE = tam giác DBE (c.g.c) - đpcm

c)

Vì tam giác ABE = tam giác DBE (câu b)

=> AE = DE

Xét tg AEF ⊥ tại A; tg DEC ⊥ tại D:

AE = DE (c/m trên)

g AEF = g DEC (đối đỉnh)

=> tg AEF = tg DEC (cgv - gn) - đpcm

=> EF = EC 

d)

Do tam giác AEF = tam giác DEC (câu c)

=> AE = DE

=> E ∈ đường trung trực của AD (1)

Lại do AB = BD (gt)

=> B ∈ đường trung trực của AD (2)

Từ (1) và (2) => BE là đường trung trực của AD. - đpcm

18 tháng 4 2021

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8 tháng 8 2016

a) dùng pyta go

b) = nhau theo trường hợp cạnh huyền cạnh góc vuông

c) dựa vào kết quả câu b =>tam giác AEF=tam giác DEC

d)tam giác ABD cân có BE là phân giác =>đpcm

25 tháng 3 2022

a) Áp dụng định lí Pytago vào ΔABC vuông tại A, ta được:

BC2=AB2+AC2

⇔BC2=32+42=25=52

sorry bt mỗi câu a hoi

gianroi

25 tháng 3 2022

ok nha đợi minh một lát

a: Xét ΔBAE vuông tại A và ΔBDE vuông tại D co

BE chung

BA=BD

=>ΔBAE=ΔBDE

b: BA=BD

EA=ED

=>BE là trung trực của AD

c: Xét ΔBDM vuông tại D và ΔBAC vuông tại A có

BD=BA

góc B chung

=>ΔBDM=ΔBAC

=>BM=BC

=>ΔBMC cân tại B

16 tháng 5 2023

Cảm ơn nhiềuu ạ yeu

a) tam giác ABC vuông tại A

=>  AB2 + AC2 = BC2

=> 52   +    72  = BC2

=> BC2 = 25 + 49 = 74

=> BC = \(\sqrt{74}cm\)

hình như bn ghi sai đề rùi làm sao làm bài b) !!!!!!!1

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