#### Question

Evaluate the following:

cos 80° cos 20° + sin 80° sin 20°

#### Solution

\[\cos80^\circ\cos20^\circ + \sin80^\circ \sin20^\circ\]

\[ = \cos\left( 80^\circ - 20^\circ \right) \left[\text{ Using }\cos A \cos B + \sin A \sin B = \cos\left( A - B \right) \right]\]

\[ = \cos60^\circ = \frac{1}{2}\]

Is there an error in this question or solution?

Solution Evaluate the Following: Cos 80° Cos 20° + Sin 80° Sin 20° Concept: Trigonometric Functions of Sum and Difference of Two Angles.