Giải thích các bước giải:
\(\begin{array}{l}
1,\\
A = \cos \left( { - 315^\circ } \right).\sin 765^\circ \\
= \cos \left( {45^\circ - 360^\circ } \right).\sin \left( {45^\circ + 2.360^\circ } \right)\\
= \cos 45^\circ .\sin 45^\circ = \frac{{\sqrt 2 }}{2}.\frac{{\sqrt 2 }}{2} = \frac{1}{2}\\
2,\\
B = \sin 32^\circ .\sin 148^\circ - \sin 302^\circ .\sin 122^\circ \\
= \sin 32^\circ .\sin \left( {180^\circ - 148^\circ } \right) - \sin \left( { - 58^\circ + 360^\circ } \right).\sin \left( {180^\circ - 58^\circ } \right)\\
= \sin 32^\circ .\sin 32^\circ + \sin 58^\circ .\sin 58^\circ \\
= {\sin ^2}32^\circ + {\sin ^2}58^\circ \\
= {\sin ^2}32^\circ + {\left( {\cos \left( {90^\circ - 58^\circ } \right)} \right)^2}\\
= {\sin ^2}32^\circ + {\cos ^2}32^\circ = 1\\
3,\\
C = \sin 810^\circ .\cos 540^\circ + \tan 135^\circ .\cot 585^\circ \\
= \sin \left( {90^\circ + 2.360^\circ } \right).\cos \left( {180^\circ + 360^\circ } \right) + \tan \left( { - 45^\circ + 180^\circ } \right).\cot \left( {45^\circ + 3.180^\circ } \right)\\
= \sin 90^\circ .\cos 180^\circ - \tan 45^\circ .\cot 45^\circ \\
= 1.\left( { - 1} \right) - 1.1 = - 2
\end{array}\)
