a) \(\eqalign{S_{ABC}} = \eqalign{{BB'.AC} \over 2} = \eqalign{{CC'.AB} \over 2}\)
\(\eqalign{ & \Rightarrow BB'.AC = CC'.AB \cr & \Rightarrow {{BB'} \over {CC'}} = {{AB} \over {AC}} \cr} \)
b) Nếu \(AB < AC\) \( \Rightarrow \eqalign{{AB} \over {AC}} < 1\)
\( \Rightarrow \eqalign{{BB'} \over {CC'}} < 1 \Rightarrow BB' < CC'\)
