$\widehat{xOx'}-\widehat{xOy'}=20^o$
mà $\widehat{xOx'}+\widehat{xOy'}=180^o$
$→\widehat{xOy'}=(180^o-20^o):2=80^o$
$→\widehat{xOx'}=180^o-80^o=100^o$
mà $\widehat{xOx'}+\widehat{x'Oy}=180^o$
$→\widehat{x'Oy}=180^o-100^o=80^o$
Vì $O∈x'y'$
$→\widehat{x'Oy'}=180^o$