Xét ht $ABCD(AB//CD)$ có: $\widehat{A}+\widehat{D}=180^o$ $⇒20^o+\widehat{D}+\widehat{D}=180^o$ $⇒2\widehat{D}=160^o$ $⇒\widehat{D}=80^o$ $⇒\widehat{A}=20^o+\widehat{D}=20^o+80^o=100^o$ $\widehat{A}+\widehat{C}=160^o$ $⇒\widehat{C}=160^o-\widehat{A}=160^o-100^o=60^o$ $\widehat{B}+\widehat{C}=180^o$ $⇒\widehat{B}=180^o-\widehat{C}=180^o-60^o=120^o$ Vậy số các góc $A,B,C,D$ của ht lần lượt là $100^o;120^o;60^o;80^o$
