Giải thích các bước giải:

a.Xét $\Delta ABD,\Delta HBE$ có:

$\widehat{BAD}=\widehat{BHE}(=90^o)$

$\widehat{ABD}=\widehat{HBE}$

$\to \Delta ABD\sim\Delta HBE(g.g)$

b.Xét $\Delta BHA,\Delta ABC$ có:

Chung $\hat B$

$\widehat{BHA}=\widehat{BAC}(=90^o)$

$\to \Delta BHA\sim\Delta BAC(g.g)$

$\to \dfrac{BH}{AB}=\dfrac{BA}{BC}$

$\to BA^2=BH.BC$