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Do AH
vuong goc voi BC nen pt AH co dang x+2y+m=0, do AH di qua H(1;-1) nen pt AH la:
x+2y+1=0.
Do A  AH nen A co toa do A(-1-2a; a)
Do C BC nen C co toa do C(c; 2c+1).
Do E la
trung diem cua AC nen co:
-1-2a +
c= 2. (-1) va a + 2c+1 = 2. 2
ó c-2a = -1 va a+2c = 3
ó c=1 va a=1
ó A(-3; 1) va C(1; 3)
Co BH vuong
goc voi AC va di qua Hoàn toàn nen BH co pt: 2x+y-1=0.
Toa do
diem B la nghiem cua he 2x+y-1=0 va 2x-y+1=0
ó B(0,1).
Vay toa
do 3 dinh cua tam giac la: A(-3; 1), B(0,1), C(1; 3).
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