Gấp! Tks m.n

H

hiendang241

ádfghjk

vì DE//BC nên $\frac{DE}{BC}$=$\frac{AD}{AB}$(ta-let)(1)
MN//BC nên $\frac{MN}{BC}$=$\frac{AM}{AB}$(ta-let)(2)
1,2 \Rightarrow $\frac{DE+MN}{BC}$=$\frac{AD+AM}{AB}$
mà AD+AM=AD+AD+DM =AD+BM+DM=AB(DA=DM)
\Rightarrow$\frac{AD+AM}{AB}$=1
\Rightarrow $\frac{DE+MN}{BC}$=1\Rightarrow DE+MN=BC \Rightarrow cố định do BC cố định
 
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