$Ta\text{có}:\widehat{O1}+ \widehat{O2}=180^{o}(hai\text{góc}\text{kề}\text{bù})$
$và \widehat{O1}=3\widehat{O2}$
$⇔\frac{\widehat{O1}}{3}=\frac{\widehat{O2}}{1}$
$Áp\text{dụng}\text{tính}\text{chất}\text{dãy}\text{dãy}\text{tỉ}\text{số}\text{bằng}\text{nhau}.Ta\text{có}:$
$\frac{\widehat{O1}}{3}=\frac{\widehat{O2}}{1}=\frac{\widehat{O1}+\widehat{O2}}{3+1}=\frac{180^{o}}{4} =45^{o}$
$⇔\widehat{O}=135^{o};\widehat{O2}=45^{o}.$
$Vì\widehat{O1}và \widehat{O3} là\text{hai}\text{đối}\text{đỉnh}$
$⇒\widehat{O1}=\widehat{O3}mà\widehat{O1}=135^{o}⇒\widehat{O3}=135^{o}.$
$Chứng\text{minh}\text{tương}\text{tự}⇒\widehat{O2}=\widehat{O4}=45^{o}.$
$Vậy \widehat{O1}=\widehat{O3}=135^{o};\widehat{O2}=\widehat{O4}=45^{o}.$
