a. Ta có:
$\angle OBA=\angle OCA=90^o \Rightarrow ABOC$ nội tiếp.
b. Ta có:
$\angle ABD=\angle AEB \Rightarrow \Delta ABD\sim\Delta AEB \Rightarrow \dfrac{AB}{AE}=\dfrac{AD}{AB} \Rightarrow AB^2=AD.AE$
$\angle AHB=\angle ABO \Rightarrow \Delta AHB\sim\Delta ABO \Rightarrow \dfrac{AH}{AB}=\dfrac{AB}{AO} \Rightarrow AB^2=AH.AO$
Từ đó suy ra: $AH.AO=AD.AE$