Đáp án+Giải thích các bước giải:
$\widehat{A}-\widehat{B}=20^o\\=>\widehat{A}=20^o+\widehat{B}$
$\text{Vì góc A và B phụ nhau }\\\text{nên $\widehat{A}+\widehat{B}=90^o$}\\\left(20^o+\widehat{B}\right)+\widehat{B}=90^o\\20^o+\widehat{B}+\widehat{B}=90^o\\20^o+2\widehat{B}=90^o\\2\widehat{B}=90^o-20^o\\2\widehat{B}=70^o\\\widehat{B}=70^o:2\\\widehat{B}=35^o\\=>\widehat{A}=20^o+\widehat{B}=20^o+35^o=55^o$
