a) [TEX]BC^2=AD^2=AI.AH[/TEX](hệ thức trong tam giác vuông)
b) [TEX]\widehat{BAC}=\widehat{ABD}=\widehat{AKB} \Rightarrow \Delta BAC \sim \Delta BKA \Rightarrow \frac{BA}{BC}=\frac{BK}{BA} \Rightarrow BK.BC=BA^2[/TEX]
c) [TEX]\widehat{DAI}=\widehat{ABD}=\widehat{ACD} \Rightarrow \Delta DAI \sim \Delta DCA \Rightarrow \frac{DA}{DI}=\frac{DC}{DA} \Rightarrow DI.DC=DA^2=BC^2[/TEX]