Do xx'//zz' nên $\widehat{ACz}=\widehat{CAx'}$ (hai góc so le trong)
Do yy'//zz' nên $\widehat{BCz}=\widehat{CBy'}$ (hai góc so le trong)
Do hai góc ACz vfa BCz là hai góc kề nhau nên $\widehat{ACB}=\widehat{ACz}+\widehat{BCz}=\widehat{CAx'}+\widehat{CBy'}$