Cho tg ABC vuông tại A có AB = 6cm, AC = 8cm, đường cao AH. 1)Tính BC và AH. 2)Kẻ HE vg góc AB tại E, HF vg góc AC tại F. Cm tg AEH đồng dạng tg AHB. 3)Cm AH2 = AF.AC 4)Cm tg ABC đồng dạng tg AFE. 5) Tính diện tích tứ giác BCFE. 6)Tia phân giác của góc BAC cắt EF ,BC lần lượt tại I và K . Chứng minh KB.IE = KC.IF mk cần rất gấp

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