Bài 19:
Kẻ Bz//Ax//Cy
Ta có: \(\widehat{A}+\widehat{ABC}+\widehat{C}\)
\(=\widehat{A}+\widehat{ABz}+\widehat{CBz}+\widehat{C}\)
\(=180^0+180^0\)
\(=360^0\)
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