Giúp em với ạ Cho tứ diện OABC có OA,OB,OC đôi một vuông góc và OA=OB=OC=a a) Chứng minh các mặt phẳng OBC, OAC, OAB đôi một vuông góc với nhau b) Gọi M là trung điểm của BC, chứng minh rằng (ABC) vuông góc với (OAM)

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