a/ Xét $ΔADB$ và $ΔAEC$:
$\widehat{A}$: chung
$\widehat{ADB}=\widehat{AEC}$ ($=90^\circ$)
$→ΔABD\backsim ΔAEC(g-g)$
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b/ $ΔABD\backsim ΔAEC$
$→\dfrac{AB}{AD}=\dfrac{AC}{AE}$
$→AB.AE=AD.AC$
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c/ $\dfrac{AB}{AD}=\dfrac{AC}{AE}$
$→\dfrac{AD}{AE}=\dfrac{AB}{AC}$
$→DE//BC$ (ĐL Talet đảo)