a)
[tex]\left\{\begin{matrix} \widehat{ADB}+\widehat{DAC}=90^{o} & & \\ \widehat{DAB}+\widehat{DAx}=90^{o} & & \end{matrix}\right. \Rightarrow \widehat{ADB}=\widehat{DAB} (\widehat{DAC}=\widehat{DAx})\Rightarrow \Delta ABD[/tex] cân
b) ta có [tex] \Delta OAE[/tex] cân tại O
[tex]\Rightarrow \widehat{AEO}=\widehat{EAO}[/tex]
mà [tex] \widehat{ADB}=\widehat{EAO}[/tex]
[tex]\Rightarrow \widehat{AEO}=\widehat{ADB}[/tex]
[tex]\Rightarrow OE//DB [/tex]
c)
[tex]\widehat{AEB}=\widehat{ACB}=90^{o}\Rightarrow \left\{\begin{matrix} EB\perp AD & & \\ AC\perp DB & & \end{matrix}\right. \Rightarrow[/tex] I là trực tâm [tex] \Delta ABD[/tex]
[tex]\Rightarrow DI\perp AB[/tex]