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Question
Evaluate the following:
cos 47° cos 13° − sin 47° sin 13°
Solution
\[ \cos47^\circ\cos13^\circ - \sin47^\circ \sin13^\circ\]
\[ = \cos\left( 47^\circ + 13^\circ \right) \left[\text{ Using }\cos A \cos B - \sin A \sin B = \cos\left( A + B \right) \right]\]
\[ = cos 60^\circ = \frac{1}{2}\]
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