a.
Xét \(\Delta AMB;\Delta DMC\) có :
\(MA=MD\left(gt\right)\\ \widehat{AMB}=\widehat{DMC}\left(đ^2\right)\\ MB=MC\left(gt\right)\\ \Rightarrow\Delta AMB=\Delta DMC\left(c-g-c\right)\)
b.
\(\Delta AMB=\Delta DMC\\ \Rightarrow\widehat{D}=\widehat{MAB}\)
=> AB // DC
\(\Rightarrow\widehat{ACD}+\widehat{BAC}=180^0\left(TCP\right)\\ \Rightarrow\widehat{ACD}=90^0\\ \Rightarrow AC\perp DC\)