a, Xét \(\Delta ABEv\text{à}\Delta ACF\)
\(AEB=\text{AF}C\left(=90^o\right)\)
\(BAE=FAC\) (góc chung)
\(\Rightarrow\Delta ABE~\Delta ACF\left(g.g\right)\)
b,Từ \(\Delta ABE~\Delta ACF\) (chứng minh trên)
\(\Rightarrow\dfrac{AB}{AC}=\frac{AE}{\text{AF}}\Rightarrow\dfrac{\text{AF}}{AC}=\dfrac{AE}{AB}\)
Xét \(\Delta AEFva\Delta ABC\)
\(\dfrac{AF}{AC}=\dfrac{AE}{AB}\left(cmt\right)\)
\(EAF=BAC\) (Góc chung)
\(\Rightarrow\Delta AEF~\Delta ABC\left(c.g.c\right)\)
\(\Rightarrow\dfrac{AE}{AB}=\dfrac{\text{EF}}{BC}\Rightarrow AE.BC=AB.\text{EF}\)