\(\begin{array}{l}
a)\,\,\,Ta\,\,co:\,\,\angle EDB = {120^0}\,\,\,\,\left( {hai\,\,goc\,\,doi\,\,\,dinh} \right)\\
\Rightarrow \angle EDB + \angle DBC = {120^0} + {60^0} = {180^0}\\
Ma\,\,hai\,\,goc\,\,\,la\,\,\,hai\,\,goc\,\,\,trong\,\,cung\,\,phia.\\
\Rightarrow ED//BC.\\
b)\,\,Vi\,\,ED//BC\,\,\left( {cmt} \right)\\
Ma\,\,EC \bot BC\\
\Rightarrow EC \bot BC\,\,\,\left( {tu\,\,vuong\,\,goc\,\,\,den\,\,\,song\,\,song} \right).
\end{array}\)
