a, Ta có: [TEX]AM=CM;BM=DM ; \widehat{M_1}=\widehat{M_2} \Rightarrow \Delta MAB=\Delta MCD (c.g.c) \Rightarrow AB=CD[/TEX]
( 2 cạch tương ứng)
b, Vì [TEX]\Delta MAB=\Delta MCD \Rightarrow \widehat{BAM}=\widehat{DCM}[/tex]
( 2 góc tương ứng) [tex] \Rightarrow AB//CD[/TEX]
(2 góc so le trong = nhau)
c,