Xét $ΔABC$ có:
+) $\widehat{A}+\widehat{B}+\widehat{C}=180^o$
+) $100^o+\widehat{B}+\widehat{C}=180^o$
$→\widehat{B}+\widehat{C}=80^o$
Mà: $\widehat{B}-\widehat{C}=50^o$
Ta có: $\widehat{B}+\widehat{C}=\widehat{C}+50^o+\widehat{C}=80^o$
$→\widehat{C}+\widehat{C}=2\widehat{C}=30^o$
$→\widehat{C}=15^o$
Lại có: $100^o+\widehat{B}+15^o=180^o$
$→\widehat{B}=65^o$