Qua B kker đường thẳng song song với AC cắt AD tại H

=> BH vuông AB

Xét tam giác ABH và tam giác CAM 

Có \(\widehat{ABH}=\widehat{CAM}=90^o\)

AB =AC ( ytam giác ABC cân)

\(\widehat{BAH}=\widehat{ACM}\)( cùng phụ với góc AMC)

=> Tam giác ABH=CAM

=> BH=MA

Vì BH//AC theo định lí thales

\(\frac{BD}{DC}=\frac{BH}{AC}=\frac{AM}{AB}=\frac{1}{2}\)

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CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCGCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC

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a) Xét \(\Delta ABC\)và\(\Delta ADE\):

AB=AD(gt)

\(\widehat{BAC}=\widehat{DAE}=90^o\)

AC=AE(gt)

=> \(\Delta ABC=\Delta ADE\left(c-g-c\right)\)

=> BC=DE ( 2 cạnh tương ứng)

=> Đpcm

b) Ta có \(\Delta ABD\)vuông cân tại A

=> \(\widehat{ABD}=\widehat{ADB}=\frac{\widehat{DAB}}{2}=\frac{90^o}{2}=45^o\)

\(\Delta AEC\)vuông cân tại A

=> \(\widehat{AEC}=\widehat{ACE}=\frac{\widehat{EAC}}{2}=\frac{90^o}{2}=45^o\)

=> \(\widehat{BDA}=\widehat{ECA}=45^o\)

Mà 2 góc này ở vị trí so le trong

=> BD//CE

=> Đpcm

c) Sửa đề: Kẻ dường cao AH của tam giác ABC cắt DE tại M. Vẽ đường thẳng qua A và vuông góc với MC cắt BC tại N. Chứng minh rằng CA vuông góc với NM

Gọi giao điể của NA và MC là I

Xét \(\Delta NMC\)có:

\(\hept{\begin{cases}NI\perp MC\\MH\perp NC\end{cases}}\)

Mà 2 đường cao này cắt nhau tại A

=> A là trực tâm của \(\Delta MNC\)

=> \(CA\perp NM\)

=> Đpcm

d) Ta có: \(\widehat{ADM}=\widehat{ABC}\left(\Delta ADE=\Delta ABC\right)\)

=> \(\widehat{ADM}+\widehat{AED}=\widehat{ABC}+\widehat{BAH}=90^o\)

=> \(\widehat{AED}=\widehat{BAH}\) Mà \(\widehat{BAH}=\widehat{MAE}\left(đđ\right)\)

=> \(\widehat{AED}=\widehat{MAE}\)

=> \(\Delta MAE\)cân tại M

=> MA=ME (1)

Lại có: \(\widehat{AED}=\widehat{ACB}\Rightarrow\widehat{AED}+\widehat{ADE}=\widehat{ACB}+\widehat{CAH}=90^o\)

=> \(\widehat{ADE}=\widehat{CAH}\)

Mà \(\widehat{CAH}=\widehat{DAM}\left(đđ\right)\)

=> \(\widehat{ADE}=\widehat{DAM}\)

=> \(\Delta DAM\)cân tại M

=> MD=MA (2)

Từ (1) và (2)

=> MA=MD=ME

=> \(MA=\frac{1}{2}DE\)

=> Đpcm

P/s: Thật ra định làm tắt cho bạn tự suy luận, nhưng sợ bạn ko hiểu nên thoi, mỏi cả tay:>>>

a: Xét tứ giác ADHE có

góc ADH=góc AEH=góc DAE=90 độ

nên ADHElà hình chữ nhật

=>góc AED=góc AHD=góc ABC

Ta có: ΔABC vuông tại A

mà AM là trung tuyến

nên MA=MC=MB

=>góc MAC=góc MCA

=>góc MAC+góc AED=90 độ

=>AM vuông góc với DE

b: HE//AB

=>HN//AB

mà góc NAB=góc HBA

nên NHBA là hình thang cân

=>góc ANB=góc AHB=90 độ

=>BN vuông góc với AM

=>BN//DE