Kẻ xy // BC, ta có:
Vì xy // BC => \(\hept{\begin{cases}\widehat{xAB}=\widehat{B}\\\widehat{yAC}=\widehat{C}\end{cases}}\)(so le trong)
Mà \(\widehat{xAB}+\widehat{A}+\widehat{yAC}=180^o\)
\(\Rightarrow\widehat{B}+\widehat{A}+\widehat{C}=180^o\)
Vậy...
ABCxy
Kẻ xy // BC, ta có:
Vì xy // BC => {
^xAB=^B |
^yAC=^C |
(so le trong)
Mà ^xAB+^A+^yAC=180o
⇒^B+^A+^C=180o
Vậy...