a) Ta có: Ax⊥MN,Cy⊥MN
=> Ax//Cy
b) Ta có: Bz//Ax
\(\Rightarrow\widehat{ABz}=\widehat{BAx}=60^0\)(so le trong)
c) Ta có: Bz//Ax,Ax//Cy
=> Bz//Cy
\(\Rightarrow\widehat{CBz}=\widehat{BCy}=30^0\)(so le trong)
\(\Rightarrow\widehat{ABC}=\widehat{ABz}+\widehat{CBz}=60^0+30^0=90^0\)
=> BA⊥BC