Câu 21. Trong các khẳng định sau, khẳng định nào đúng?

\(\cos \left( {\frac{\pi }{6} - \frac{\pi }{3}} \right) = \cos \frac{\pi }{6}.\cos \frac{\pi }{3} - \sin \frac{\pi }{6}.\sin \frac{\pi }{3}\).

\(\cos \left( {\frac{\pi }{4} - \frac{\pi }{3}} \right) = \cos \frac{\pi }{4}.\cos \frac{\pi }{3} + \sin \frac{\pi }{4}.\sin \frac{\pi }{3}\).

\(\cos \left( {\frac{\pi }{6} - \frac{\pi }{3}} \right) = \sin \frac{\pi }{6}.\cos \frac{\pi }{3} - \cos \frac{\pi }{6}.\sin \frac{\pi }{3}\).

\(\cos \left( {\frac{\pi }{4} - \frac{\pi }{3}} \right) = \sin \frac{\pi }{4}.\cos \frac{\pi }{3} + \cos \frac{\pi }{4}.\sin \frac{\pi }{3}\).