\(a,\left\{{}\begin{matrix}\widehat{ABC}=\widehat{BCD}\left(AB\text{//}CD\right)\\\widehat{ACB}=\widehat{CBD}\left(AC\text{//}BD\right)\\BC\text{ chung}\end{matrix}\right.\Rightarrow\Delta ABC=\Delta DCB\left(g.c.g\right)\\ b,\Delta ABC=\Delta DCB\\ \Rightarrow AB=CD;AC=BD\Rightarrow\dfrac{1}{2}AC=\dfrac{1}{2}BD\\ \Rightarrow AM=ND\\ \left\{{}\begin{matrix}AB=CD\\\widehat{BAC}=\widehat{BDC}\left(\Delta ABC=\Delta DCB\right)\\AM=ND\end{matrix}\right.\Rightarrow\Delta ABM=\Delta DCN\left(c.g.c\right)\)
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