bạn có thể làm như sau:Cho $\alpha +\beta +\gamma =\pi$ Để:
$cos^{2}\alpha +cos^{2}\beta +cos^{2}\gamma = a + b.cos\alpha .cos\beta .cos\gamma$
Thì a= ?, b=?
Ta có
$VT= cos^{2}\alpha +\frac{1+cos2\beta }{2}+\frac{1+cos2\gamma }{2}$
$= cos^{2}\alpha +1+cos(\beta +\gamma )cos(\beta -\gamma )$
$= 1- cos\alpha (cos(\beta +\gamma )+cos(\beta -\gamma ))$
$= 1- 2cos\alpha. cos\beta. cos\gamma$
Vậy $a=1$,$b=-2$