cos(П/6+b)=cosbcosП/6-sinb*sinП/6=cosbsqrt(3)/2-sqrt(3)/4=
cosb<0 cos^2b=1-sin^2b=1/4
cosb=-1/2
= (sqrt(3)/2)*(-1/2-1/2)=-sqrt(3)/2
cos(П/6+b)=cosbcosП/6-sinb*sinП/6=cosbsqrt(3)/2-sqrt(3)/4=
cosb<0 cos^2b=1-sin^2b=1/4
cosb=-1/2
= (sqrt(3)/2)*(-1/2-1/2)=-sqrt(3)/2