Trên nửa mặt phẳng bờ AB chứa C, kẻ \(BD//Ax\), Ta có:
\(\widehat{xAB}=\widehat{ABD}=100^o\)(2 góc so le trong)
Do tia \(BC\)nằm giữa 2 tia \(BA\)và \(BD\)
\(\Rightarrow\widehat{ABC}+\widehat{CBD}=\widehat{ABD}\)
Thay số: \(40^o+\widehat{CBD}=100^o\)
\(\Rightarrow\widehat{CBD}=100^o-40^o=60^o\)
+) Do\(\hept{\begin{cases}BD//Ax\\Ax//Cy\left(gt\right)\end{cases}}\)
\(\Rightarrow BD//Cy\)(Tính chất bắc cầu)
\(\Rightarrow\widehat{yCB}+\widehat{CBD}=180^o\)
Thay số: \(\Rightarrow\widehat{yCB}+60^o=180^o\)
\(\Rightarrow\widehat{yCB}=180^o-60^o=120^o\)
Vậy, \(\widehat{BCy}=120^o\)
