b) Chứng minh AF.AB = AE.AC..

b) Xét hai tam giác DAEB và DAFC có:

$\left.\begin{array}{c}\widehat{EAB}=\widehat{FAC}\left(\widehat{A}chung\right)\\ \widehat{AEB}=\widehat{ÀC}\left(=90°\right)\end{array}\right\}\Rightarrow \Delta AEB\backsim \Delta AFC\left(g.g\right)$

$\Rightarrow \frac{AE}{AF}=\frac{AB}{AC}\iff AE.AC=AF.AB$ (đpcm)