Tính giá trị biểu thức $P = \cos {30^ \circ }\cos {60^ \circ } - \sin {30^ \circ }\sin {60^ \circ }.$ 

$\sin \left( {180^\circ  - \alpha } \right) =  - \cos \alpha .$

$\sin \left( {180^\circ  - \alpha } \right) =  - \sin \alpha .$

$\sin \left( {180^\circ  - \alpha } \right) = \sin \alpha .$

$\sin \left( {180^\circ  - \alpha } \right) = \cos \alpha .$

$\sin \alpha  =  - \cos \beta .$      

$\cos \alpha  = \sin \beta .$

$\tan \alpha  = \cot \beta .$

$\cot \alpha  = \tan \beta .$

$\sin 90^\circ  < \sin 150^\circ .$

$\sin 90^\circ 15' < \sin 90^\circ 30'.$

$\cos 90^\circ 30' > \cos 100^\circ .$

$\cos 150^\circ  > \cos 120^\circ .$

$1.$

$\sqrt 2 .$

$\sqrt 3 .$

$0.$

$S = 0.$        

$S = 1.$

$S = 2.$

$S = 4.$