cho goc xOy khac goc bet ,Ot la phan giac. tren Ot lay H qua H ke duong thang vuong goc Ot cat Ox tai A,Oy tai B.
chung minh a)AH=BH
b)tren Ax lay C,tren By lay D sao cho AC=BD.CHUNG MINH AD=BC
c)chung minh AB//CD
d)chung minh AB,BC,OH cung di qua mot diem