Câu 3.
Vì $\widehat{AOB}$ và $\widehat{BOC}$ là 2 góc kề nhau
$\Rightarrow \widehat{AOB}+\widehat{BOC}=\widehat{AOC}$
mà $\widehat{AOC}$ = 80o $\Rightarrow \widehat{AOB}+\widehat{BOC}={{80}^{o}}$
Có: $\widehat{AOB}=\frac{1}{5}\widehat{AOC}=\frac{1}{5}{{.80}^{o}}={{16}^{o}}$
Suy ra
${{16}^{o}}+\widehat{BOC}={{80}^{o}}$
$\Rightarrow \widehat{BOC}={{80}^{o}}-{{16}^{o}}={{64}^{o}}$