a) Xet △DCE va △DCF co:
\(\left\{{}\begin{matrix}DE=DF\left(tgDEFcan\right)\\chungDC\\EC=CF\end{matrix}\right.\)
=> △DCE=△DCF (c.c.c)
b) △DCE=△DCF => \(\widehat{DCE}=\widehat{DCF}\)
va C ∈ EF
=> \(\widehat{DCE}=\widehat{DCR}=90\)
=> DC⊥EF
c) Xet △EAC va △BFC co:
\(\left\{{}\begin{matrix}\widehat{E}=\widehat{F}\\EC=CF\\\widehat{EAC}=\widehat{CBF}\end{matrix}\right.\)
=> △EAC=△BFC ( g.c.g) => AC=BC
Xet △DAC va △DBC co:
\(\left\{{}\begin{matrix}DA=DB\\\widehat{DAC}=\widehat{DBC}=90\\AC=BC\end{matrix}\right.\)
=> △DAC=△DBC (c.g.c)
