Cho hình thang ABCD ( có diện tích bằng 612 cm2. Biết rằng độ dài cạnh đáy CD gấp hai lần độ dài cạnh đáy AB. a) Tính diện tích tam giác ABC và tam giác ACD. b) Gọi O là giao điểm của hai đường chéo AC và BD. Tính diện tích tam giác AOB. Cần câu b thôi ạ, cần gấp ạ

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