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a ) Xét ∆ BDM và ∆ CEM có :
∠D = ∠E = 900 (gt)
BM = MC (gt)
∠M1 = ∠M2 ( đối đỉnh )
=> ∆ BDM = ∆ CEM ( CH - GN )
=> BD = CE ; DM = EM ( Cạnh tưng ứng )
b ) Trên tiam AM lấy điểm I sao cho AM = MI
Xét ∆ ABM và ∆ ICM có :
AM = MI (gt)
∠M1 = ∠M2 ( đối đỉnh )
BM = MC (gt)
=> ∆ ABM = ∆ ICM (c - g - c)
=> AB = CI ( Cạnh tưng ứng )
∆ ACI có AC + CI > AI ( bđt tam giác)
Mà AM = 1/2AI => AC + CI > 2AM
Mà AB = CI (cm trên) => AB + AC > 2AM (đpcm)
a) Xét tam giác vuông ADB và tam giác vuông ACE có:
Góc A chung
AB = AC (gt)
\(\Rightarrow\Delta ABD=\Delta ACE\) (Cạnh huyền - góc nhọn)
b) Do \(\Delta ABD=\Delta ACE\Rightarrow AD=AE\)
Xét tam giác vuông AEH và tam giác vuông ADH có:
Cạnh AH chung
AE = AD (cmt)
\(\Rightarrow\Delta AEH=\Delta ADH\) (Cạnh huyền - cạnh góc vuông)
\(\Rightarrow HE=HD\)
c) Xét tam giác ABC có BD, CE là đường cao nên chúng đồng quy tại trực tâm. Vậy H là trực tâm giác giác.
Lại có AM cũng là đường cao nên AM đi qua H.
d) Xét các tam giác vuông EBC và EAC, áp dụng định lý Pi-ta-go ta có:
\(BC^2=EB^2+EA^2;AC^2=EA^2+EC^2\)
Tam giác ABC cân tại A nên AB = AC hay \(AB^2=AC^2\)
Vậy nên \(AB^2+AC^2+BC^2=2AC^2+BC^2=2\left(EA^2+EC^2\right)+EB^2+EC^2\)
\(=3EC^2+2EA^2+BC^2\).
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CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCGCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC
a) Xét ∆ vuông BDM và ∆ vuông MCE ta có :
BM = MC (gt)
DMB = CME ( đối đỉnh)
=> ∆BDM = ∆MCE ( ch-gn)
b) => BD = EC ( 2 góc tương ứng
Ta có : DM < BM ( Trong ∆ vuông cạnh huyền luôn luôn lớn hơn cạnh góc vuông )
Mà BM = MC
=> DM < MC ( trái đk đề bài )