Đáp án:
a) Có $\widehat{UBy}$+$\widehat{yBA}$`=180^o`(2 góc kề bù)
`⇒`$\widehat{yBA}$`=180^o-`$\widehat{UBy}$`=180^o-60^o=120^o=`$\widehat{BAx}$
Mà `2` góc này ở vị trí SLT `⇒By` // `Ax`
`b)` Kẻ `Az`//`By`//`Ct`
`Az`//`By⇒`$\widehat{UBy}$`=`$\widehat{BAz}$`=60^o(2` góc ĐV `)`
`Az`//`Ct⇒`$\widehat{zAC}$`+`$\widehat{ACt}$`=180^o(2` góc TCP `)`
`⇒`$\widehat{zAC}$`=180^o-`$\widehat{ACt}$`=180^o-150^o=30^o`
Có $\widehat{BAC}$`=`$\widehat{BAz}$+$\widehat{zAC}$`=60^o + 30^o=90^o`
`⇒AB⊥AC`
