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Cho tứ diện S.ABC có đáy ABC là tam giác vuông tại A biết SB vuống góc ABC,SB=AB.Gọi H,I,K lần lượt là trung điểm của SA,SB,BC chứng minh rằng: a,AC vuông goc SAB c,KI vuông góc SA b,BH vuông góc SAC d,AB vuông góc IH
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