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# Evaluate the Following: Cos 80° Cos 20° + Sin 80° Sin 20° - CBSE (Science) Class 11 - Mathematics

ConceptTrigonometric Functions of Sum and Difference of Two Angles

#### Question

Evaluate the following:
cos 80° cos 20° + sin 80° sin 20°

#### Solution

$\cos80^\circ\cos20^\circ + \sin80^\circ \sin20^\circ$
$= \cos\left( 80^\circ - 20^\circ \right) \left[\text{ Using }\cos A \cos B + \sin A \sin B = \cos\left( A - B \right) \right]$
$= \cos60^\circ = \frac{1}{2}$

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#### APPEARS IN

RD Sharma Solution for Mathematics Class 11 (2019 to Current)
Chapter 7: Values of Trigonometric function at sum or difference of angles
Ex.7.10 | Q: 7.4 | Page no. 19
Solution Evaluate the Following: Cos 80° Cos 20° + Sin 80° Sin 20° Concept: Trigonometric Functions of Sum and Difference of Two Angles.
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