giup minh voi :Cho tam giác ABC vuông tại A, có đường cao AH. Gọi D là điểm đối xứng với H qua AB, E là điểm đối xứng với H qua AC . a/ Chứng minh A là trung điểm của DE. b/ ∆DHE là tam giác gì? Vì sao ? c/ So sánh độ dài BC với BD + CE.

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