\(\dfrac{AB}{AC}=\dfrac{10}{20}=\dfrac{1}{2}\) ; \(\dfrac{AD}{AB}=\dfrac{5}{10}=\dfrac{1}{2}\)
-Xét △ABD và △ACB:
\(\widehat{BAD}=\widehat{CAB}=90^0\)
\(\dfrac{AB}{AC}=\dfrac{AD}{AB}\left(=\dfrac{1}{2}\right)\)
\(\Rightarrow\)△ABD∼△ACB (c-g-c).
\(\Rightarrow\widehat{ABD}=\widehat{ACB}\).