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

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

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

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

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

\(\sin \alpha  =  - \cos \beta .\)      

\(\cos \alpha  = \sin \beta .\)

\(\tan \alpha  = \cot \beta .\)

\(\cot \alpha  = \tan \beta .\)

\(\sin 90^\circ  < \sin 150^\circ .\)

\(\sin 90^\circ 15' < \sin 90^\circ 30'.\)

\(\cos 90^\circ 30' > \cos 100^\circ .\)

\(\cos 150^\circ  > \cos 120^\circ .\)

\(1.\)

\(\sqrt 2 .\)

\(\sqrt 3 .\)

\(0.\)

\(S = 0.\)        

\(S = 1.\)

\(S = 2.\)

\(S = 4.\)