a,
SA, SB là hai tiếp tuyến nên $\widehat{OAS}=\widehat{OBS}=90^o$
$\Rightarrow \widehat{OAS}+\widehat{OB}=90^o+90^o=180^o$
$\Rightarrow$ Tứ giác OASB nội tiếp.
b,
$\widehat{AOB}+\widehat{OAS}+\widehat{ASB}+\widehat{OBS}=360^o$
$\Rightarrow sđ\stackrel\frown{AB}= \widehat{AOB}= 360-90-90-60=120^o$