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thienbinhgirl

a,Ta có: $\Delta AEB\sim \Delta MED (g.g)\rightarrow \frac{ME}{EA}=\frac{DM}{AB}$
$\Delta CFM\sim \Delta AFB (g.g)\rightarrow \frac{MF}{FB}=\frac{MC}{AB}$
Mà DM=MC (gt) \Rightarrow $\frac{ME}{EA}=\frac{MF}{FB}$ \Rightarrow EF//AB
b, $CD=12cm \rightarrow DM=MC=6cm \rightarrow \frac{DM}{AB}=\frac{MC}{AB}=\frac{6}{7,5}=\frac{4}{5}\rightarrow \frac{EF}{AB}=\frac{4}{5}\rightarrow EF=\frac{7,5.4}{5}=6 cm$
 
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