a) $\widehat{AEF}-\widehat{BEF}=30^o$
$⇒\widehat{AEF}=30^o+\widehat{BEF}$
mà $\widehat{AEF}+\widehat{BEF}=180^o$
$⇒(30^o+\widehat{BEF})+\widehat{BEF}=180^o$
$⇒30^o+\widehat{BEF}+\widehat{BEF}=180^o$
$⇒30^o+2.\widehat{BEF}=180^o$
$⇒2.\widehat{BEF}=150^o$
$⇒\widehat{BEF}=150^o:2=75^o$
$AB//CD$
$⇒\widehat{BEF}+\widehat{EFD}=180^o$ (trong cùng phía)
$75^o+\widehat{EFD}=180^o$
$⇒\widehat{EFD}=180^o-75^o=105^o$
b) $\widehat{AEF}-\widehat{AFC}=40^o$
$⇒\widehat{AEF}=40^o+\widehat{EFC}$
mà $\widehat{AEF}+\widehat{EFC}=180^o$ (trong cùng phía)
$⇒40^o+\widehat{EFC}+\widehat{EFC}=180^o$
$40^o+2.\widehat{EFC}=180^o$
$2.\widehat{EFC}=180^o-40^o=140^o$
$⇒\widehat{EFC}=140^o:2=70^o$
$⇒\widehat{AEF}=40^o+70^o=110^o$
