Answer
$\frac{1+\sqrt3}{2\sqrt2}$.
Work Step by Step
We know that $\cos(A/2)=\pm\sqrt{\frac{1+\cos A}{2}}$.
Hence here $\cos 15=\pm\sqrt{\frac{1+\cos 30}{2}}=\pm\sqrt{\frac{1+\frac{\sqrt3}{2}}{2}}=\frac{1+\sqrt3}{2\sqrt2}$.
Trigonometry 7th Edition
by McKeague, Charles P.; Turner, Mark D.
Published by
Cengage Learning
ISBN 10:
1111826854
ISBN 13:
978-1-11182-685-7
Chapters 1-8 - Cumulative Test - Page 477: 13
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