... bai tap 3 .2) 158 (1) la hinh binh hanh la Dat OA2=a2, Ofi2=fe2, OC2=C2, OD2 = c ?2 • Dilu kien cin va du dl tfl giac A2fi2C2D2 la hinh binh hanh la : 02+ ^2 =fe2+ ^2 DatOA = a, ^ Tacd (2) = b,OC ... (h.3 .11) Ta cd : IOAI = lofil = locI = |OD| 2 SA =(SO + OA)^= SO +0A 2 -^ +2. S0.0A 2 SC =(SO + OCf = S0 +0C ^ ^ +SC =2S0 , , +2S0.0C +dA +0C _ ,2 Hinh 3 .11 ^ • +2sd(0A •! >2 + 0C) >2 '2 Ma ... phing (fl^ ta dung hinh binh hanh A2fi2C2D2 Trdn cac doan AjA2, B1B2, CjC2, DjD2 ta lin lugt liy cac dilm A, B, C, D cho AAj _ BBi _ CCi _ DDi AA2~ BB2 ~ CC2 " DD2 ~ Chflng minh ring tfl giac ABCD...