[toán 8] Chứng minh

A

angellovedevilforever

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N

nhuquynhdat

a) Xét $\Delta BCN$ có $AM//BC \Longrightarrow \dfrac{BN}{AB}=\dfrac{CN}{CM}$

Xét $\Delta CDM$ có $AN//CD \Longrightarrow \dfrac{AM}{DM}=\dfrac{MN}{CM} \Longrightarrow \dfrac{AM+DM}{DM}=\dfrac{MN+CM}{CM} \Longrightarrow \dfrac{AD}{DM}=\dfrac{CN}{CM} \Longrightarrow \dfrac{AB}{DM}=\dfrac{CN}{CM}$

$\dfrac{AB}{DM}= \dfrac{BN}{AB} \Longrightarrow AB^2=DM.BN$
 
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