Giúp pppppppppppp x2

a, Từ B kẻ tia Bz//Ax //Cy

Vì Bz // Ax \ $\displaystyle \Rightarrow \ \widehat{xAB} +\widehat{ABz} =180^{o}$ (2 góc trong cùng phía)

Vì Cy // Bz \ $\displaystyle \Rightarrow \ \widehat{zBC\ } +\widehat{BCy} \ =180^{o} \ $(2 góc trong cùng phía)

Suy ra $\displaystyle \ \widehat{xAB} +\widehat{ABz}$ +$\displaystyle \widehat{zBC\ } +\widehat{BCy} \ =360^{o}$

Hay $\displaystyle \ \widehat{xAB} +\widehat{ABC}$ $\displaystyle +\widehat{BCy} \ =360^{o}$

vì $\displaystyle \widehat{ABz} +\widehat{zBC} =\widehat{ABC}$

b,Từ B kẻ tia Bz//Ax

Ta có:

$\displaystyle  \begin{array}{{>{\displaystyle}l}}
\widehat{ABC\ } +\widehat{xAB} +\widehat{BCy} \ =\ 360^{o}\\
\widehat{ABz\ } +\widehat{zBC\ } +\widehat{xAB} +\widehat{BCy} \ =360^{o}
\end{array}$

mà $\displaystyle Bz\ //\ Ax\ \Rightarrow \ \widehat{xAB\ } +\ \widehat{ABz} =180^{o}$

$\displaystyle  \begin{array}{{>{\displaystyle}l}}
\Rightarrow \widehat{ABz\ } +\widehat{zBC\ } +\widehat{xAB} +\widehat{BCy} \ =360^{o}\\
\Rightarrow \widehat{zBC\ } +\widehat{BCy} \ =180^{o}
\end{array}$

Mà 2 góc trong cùng một phía nên Bz// Cy

$\displaystyle \Rightarrow Bz\ //\ Cy\ và\ Bz\ //\ Ax\ \ \Rightarrow \ Ax\ //\ Cy\ $