cmr a) cos$\widehat{MOA}$ +cos$\widehat{MOB}$ +cos$\widehat{MOC}$=0 ( da chung minh duoc)
b)
$cos^{2}\widehat{MOA}+cos^{2}\widehat{MOB}+cos^{2}\widehat{MOC}=\frac{3}{2}$
c)
$cos^{4}\widehat{MOA}+cos^{4}\widehat{MOB}+cos^{4}\widehat{MOC}=\frac{9}{8}$
- bbboylion yêu thích