goi H(x,y)
suy ra: vecto AH=(x-1,y-2)
taco: vecto BC=(6,-2)
vi AH la duong cao tam giac ABC
suy ra: AH vuong goc voi BC
suy ra: vecto AH nhan voi vecto BC se =0
hay: (x-1)*6+(y-2)*(-2)=0
tuong duong: 6x-2y-2=0 (1)
roi sau do lap pt canh BC
H thuoc BC nen toa do cua H la nghiem cua pt canh BC
thay toa do cua H vao pt canh BC ta duoc pt (2)
giai he pt (1) va (2), ta tim duoc toa do cua H(x,y)