Ta có:$\widehat{yBC}+\widehat{dBC}=\widehat{yBd}=180^o$
$⇒90^o+\widehat{dBC}=180^o$
$⇒\widehat{dBC}=90^o$
Hay $\widehat{B_2}=90^o$
Mà $\widehat{ABC}=\widehat{B_1}+\widehat{B_2}$
$⇒150^o=\widehat{B_1}+90^o$
$⇒\widehat{B_1}=60^o$
Ta thấy $\widehat{B_1}+\widehat{xAB}=60^o+120^o=180^o$
$⇒Ax//By$ 2 góc trong cùng phía
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