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kimtong298
2 năm trước
Cho tam giác ABC cân tại A ( góc A nhỏ hơn 90 độ ). Kẻ BD vuông góc AC (D thuộc AC), CE vuông góc AB, (E thuộc AB); BD và CE cắt nhau tại H. Chứng minh: DE song song với BC
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