Xét \(\Delta ABC:\widehat{BAC}+\widehat{B}+\widehat{C}=180^o\) (Tổng 3 góc trong tam giác).
Mà \(\widehat{B}=\widehat{C}=50^o\left(gt\right).\)
\(\Rightarrow\widehat{BAC}=80^o.\)
\(\widehat{BAC}+2\widehat{BAx}=180^o.\\ \Rightarrow80^o+2\widehat{BAx}=180^o.\\ \Rightarrow\widehat{BAx}=50^o.\)
Có: \(\widehat{B}=50^o\left(gt\right).\)
\(\Rightarrow\widehat{B}=\widehat{BAx}.\)
Mà 2 góc này ở vị trí so le trong.
\(\Rightarrow Ax//BC\left(dhnb\right).\)
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