cho tam giác ABC vuông tại A ( AB> AC ) vẽ đường tròn tâm O đường kính AB cắt BC tại H (H khác B) qua O vẽ đường thẳng song song với BC cắt AC tại E a. cho AB= 20cm, AC=15cm CM: AH vuông góc với BC và tính độ dài AH b. CM: EH là tiếp tuyến của đường tròn (O) c. Vẽ HF vuông góc với AB tại F, OE cắt AH tại K, BE cắt HF tại I. Gọi T là giao điểm của đường thẳng IK và AC. CM:IT vuông góc với AC và AT.AC=2AKbình

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