Cho tam giác ABC cân tại A, kẻ AH vuông góc BC

$$\begin{gathered} Bt2: Cho ∆ ABC\hspace{0.17em}cân tại A kẻ AH \perp BC(H\in BC ) \\ a.\mathit{C}\mathit{M}\mathbf{:}\mathbf{ }\mathit{H}\mathit{B}\mathbf{=}\mathit{H}\mathit{C} \\ \mathit{b}\mathbf{.}\mathit{K}\mathit{ẻ}\mathbf{ }\mathit{H}\mathit{D}\mathbf{\perp }\mathbf{ }\mathit{A}\mathit{B}\mathbf{ }\mathbf{(}\mathit{D}\mathbf{\in }\mathit{A}\mathit{B}\mathbf{)}\mathbf{,}\mathit{H}\mathit{E}\mathbf{\perp }\mathit{B}\mathit{C}\mathbf{(}\mathit{E}\mathbf{\in }\mathit{A}\mathit{C}\mathbf{)}\mathbf{:}\mathit{c}\mathit{m}\mathbf{ }\mathbf{∆}\mathit{H}\mathit{C}\mathit{D}\mathit{E}\mathbf{ }\mathit{c}\mathit{â}\mathit{n} \\ \mathit{c}\mathbf{.}\mathbf{ }\mathit{N}\mathit{ế}\mathit{u}\mathbf{ }\mathbf{ }\mathit{c}\mathit{h}\mathit{o}\mathbf{ }\mathit{B}\mathit{A}\mathit{C}\mathbf{ }\mathbf{=}\mathbf{120}\mathbf{ }\mathit{đ}\mathit{ộ}\mathbf{ }\mathit{t}\mathit{h}\mathit{ì}\mathbf{ }\mathbf{∆}\mathit{H}\mathit{D}\mathit{E}\mathbf{ }\mathit{t}\mathit{h}\mathit{à}\mathit{n}\mathit{h}\mathbf{ }\mathbf{∆}\mathbf{ }\mathit{g}\mathit{ì}\mathbf{?}\mathbf{ }\mathit{V}\mathit{ì}\mathbf{ }\mathit{s}\mathit{a}\mathit{o}\mathbf{?} \\ \mathit{d}\mathbf{.}\mathit{c}\mathit{m}\mathbf{ }\mathit{B}\mathit{C}\mathbf{/}\mathbf{/}\mathbf{ }\mathit{D}\mathit{E} \end{gathered}$$