a)\(\Delta ABC\) có \(\widehat{BAC}+\widehat{ABC}+\widehat{ACB}=180^o\)
=>\(\widehat{BAC}+45^o+45^o=180^o\)
=>\(\widehat{BAC}=90^o\)
b) \(\widehat{BAC}+\widehat{BAy}+\widehat{xAy}=180^o\)
=>\(90^o+\widehat{BAy}+\widehat{xAy}=180^o\)
=>\(\widehat{BAy}+\widehat{xAy}=90^o\)
Vì Ay là tia phân giác của góc BAx => \(\widehat{BAy}=\widehat{xAy}=90^o:2=45^o\)
Góc BAy và góc ABC là 2 góc so le trong mà \(\widehat{BAy}=\widehat{ABC}=45^o\)
=> Ay // BC (đpcm)
c)\(\widehat{xAy}+\widehat{HAy}+\widehat{HAC}=180^o\)
=>\(45^o+90^o+\widehat{HAC}=180^o\)
=>\(\widehat{HAC}=45^o\)
=>\(\widehat{ABC}=\widehat{HAC}=45^o\) (đpcm)