Answer
$\frac{\sqrt6}{2}$
Work Step by Step
We know that $\cos A+\cos B=2\cos\frac{A+B}{2}\cos\frac{A-B}{2}$, hence $\cos 15+\cos 75=2\cos45\cos-30=2\frac{\sqrt2}{2}\frac{\sqrt3}{2}=\frac{\sqrt6}{2}$
Trigonometry 7th Edition
by McKeague, Charles P.; Turner, Mark D.
Published by
Cengage Learning
ISBN 10:
1111826854
ISBN 13:
978-1-11182-685-7
Chapter 5 - Test - Page 316: 30
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