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Question
Evaluate the following:
cos 80° cos 20° + sin 80° sin 20°
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Solution
\[\cos80^\circ\cos20^\circ + \sin80^\circ \sin20^\circ\]
\[ = \cos\left( 80^\circ - 20^\circ \right) \left[\text{ Using }\cos A \cos B + \sin A \sin B = \cos\left( A - B \right) \right]\]
\[ = \cos60^\circ = \frac{1}{2}\]
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