Department of Pre-University Education, KarnatakaPUC Karnataka Science Class 11
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Prove That: 2 Cos 5 π 12 Cos π 12 = 1 2 - Mathematics

Sum

Prove that:

\[2\cos\frac{5\pi}{12}\cos\frac{\pi}{12} = \frac{1}{2}\]
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Solution

\[LHS = 2\left( \cos \frac{5\pi}{12} \right) \left( \cos \frac{\pi}{12} \right)\]
\[ = \cos \left( \frac{5\pi}{12} + \frac{\pi}{12} \right) + \cos \left( \frac{5\pi}{12} - \frac{\pi}{12} \right) \left[ \because 2 \cos A \cos B = \cos (A + B) + \cos (A - B) \right]\]
\[ = \cos \frac{\pi}{2} + \cos \frac{\pi}{3}\]
\[ = 0 + \frac{1}{2}\]
\[ = \frac{1}{2}\]
\[RHS = \frac{1}{2}\]
Hence, LHS = RHS

Concept: Transformation Formulae
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APPEARS IN

RD Sharma Class 11 Mathematics Textbook
Chapter 8 Transformation formulae
Exercise 8.1 | Q 2.2 | Page 6
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