`a)` Xét $\triangle$`ABD` và $\triangle$`ACE`

$\widehat{ADB}$`=`$\widehat{AEC}$`(=90^o)`

$\widehat{BAC}$ Chung

`=>` $\triangle$`ABD`$\backsim$ $\triangle$`ACE(` g`-`g`)`

`=>(AB)/(AC)=(AD)/(AE)(` ĐN `)`

`=>AB*AE=AC*AD`

`b)` Xét  $\triangle$`ADB` và  $\triangle$`ABC`

`(AB)/(AC)=(AD)/(AE)(`CM câu `a)`

$\widehat{BAC}$ Chung

`=>`$\triangle$`ADB`$\backsim$ $\triangle$`ABC(`c`-`g`-`c`)`