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Express the Following as the Sum Or Difference of Sines and Cosines: 2 Cos 7x Cos 3x - Mathematics

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Question

Express the following as the sum or difference of sines and cosines:
 2 cos 7x cos 3x

Sum
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Solution

\[2\left( \cos 7x \right) \left( \cos 3x \right)\]
\[ = \cos \left( 7x + 3x \right) + \cos \left( 7x - 3x \right) \left[ \because 2 \cos A \cos B = \cos(A + B) + \cos(A - B) \right]\]
\[ = \cos 10x + \cos 4x\]

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Chapter 8: Transformation formulae - Exercise 8.1 [Page 6]

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RD Sharma Mathematics [English] Class 11
Chapter 8 Transformation formulae
Exercise 8.1 | Q 1.4 | Page 6

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