Hình 57:
Kẻ Ez//AB//CD
\(\Rightarrow\left\{{}\begin{matrix}\widehat{AEz}+\widehat{EAB}=180^0\\\widehat{CEz}+\widehat{ECD}=180^0\end{matrix}\right.\)(trong cùng phía)
\(\Rightarrow\left\{{}\begin{matrix}\widehat{AEz}=180^0-130^0=50^0\\\widehat{CEz}=180^0-150^0=30^0\end{matrix}\right.\)
\(\Rightarrow\widehat{AEC}=50^0+30^0=80^0\)
Hình 59:
Kẻ Ez//AB//CD
\(\Rightarrow\left\{{}\begin{matrix}\widehat{ABE}=\widehat{BEz}\\\widehat{EDC}=\widehat{DEz}=40^0\end{matrix}\right.\)(so le trong)
\(\Rightarrow\widehat{ABE}=\widehat{BEz}=\widehat{BED}-\widehat{BEz}=70^0-40^0=30^0\)
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