Cho tam giác cân tại A vẽ AD vuông góc BC

$$\begin{gathered} \mathbf{cho}\mathbf{ }\mathbf{∆}\mathbf{ABC}\mathbf{ }\mathbf{cân}\mathbf{ }\mathbf{tại}\mathbf{ }\mathbf{A}\mathbf{,}\mathbf{ }\mathbf{vẽ}\mathbf{ }\mathbf{AD}\mathbf{\perp }\mathbf{BC}\mathbf{(}\mathbf{D}\mathbf{\in }\mathbf{BC}\mathbf{)}\mathbf{ } \\ \mathbf{a}\mathbf{.}\mathbf{ }\mathbf{chứng}\mathbf{ }\mathbf{minh}\mathbf{ }\mathbf{BD}\mathbf{=}\mathbf{CD} \\ \mathbf{b}\mathbf{.}\mathbf{ }\mathbf{vẽ}\mathbf{ }\mathbf{DH}\mathbf{ }\mathbf{\perp }\mathbf{AB}\mathbf{ }\mathbf{tại}\mathbf{ }\mathbf{H}\mathbf{ }\mathbf{và}\mathbf{ }\mathbf{DK}\mathbf{\perp }\mathbf{AC}\mathbf{ }\mathbf{tại}\mathbf{ }\mathbf{K}\mathbf{.}\mathbf{ }\mathbf{Chứng}\mathbf{ }\mathbf{minh}\mathbf{ }\mathbf{DH}\mathbf{=}\mathbf{DK} \\ \mathbf{c}\mathbf{.}\mathbf{ }\mathbf{chứng}\mathbf{ }\mathbf{minh}\mathbf{ }\mathbf{HK}\mathbf{\parallel }\mathbf{BC} \\ \mathbf{d}\mathbf{.}\mathbf{cho}\mathbf{ }\mathbf{AB}\mathbf{=}\mathbf{10}\mathbf{ }\mathbf{cm}\mathbf{,}\mathbf{ }\mathbf{BC}\mathbf{=}\mathbf{12}\mathbf{cm}\mathbf{.}\mathbf{ }\mathbf{Tính}\mathbf{ }\mathbf{AD} \end{gathered}$$