Bai 1 
a,
Tam giac ABC co AB = AC
=> tam giac ABC can tai A
=> AH vua la duong cao vua la duong phan giac
=> Goc BAH = goc CAH
Xet tam giac AEH va tam giac AFH ta co:
+) Goc BAH = goc CAH
+) Goc AEH = goc AFH = 90
+) canh huyen AH chung
=> Tam giac AEH = tam giac AFH ( canh huyen goc nhon)
b,
AH vua la duong cao vua la duong trung tuyen
=> AH la duong trung truc