Precalculus: Mathematics for Calculus, 7th Edition

Published by Brooks Cole
ISBN 10: 1305071751
ISBN 13: 978-1-30507-175-9

Chapter 7 - Section 7.3 - Double-Angle, Half-Angle, and Product-Sum Formulas - 7.3 Exercises - Page 561: 25



Work Step by Step

Use the half-angle formula, $\cos\frac{u}{2}=\pm\sqrt{\frac{1+\cos u}{2}}$. Note that $\frac{\pi}{12}$ is in Quadrant I, where cosine is positive, so we take the positive square root. $\cos \frac{\pi}{12}$ $=\cos \frac{\frac{\pi}{6}}{2}$ $=\sqrt{\frac{1+\cos \frac{\pi}{6}}{2}}$ $=\sqrt{\frac{1+\frac{\sqrt{3}}{2}}{2}}$ $=\sqrt{\frac{(1+\frac{\sqrt{3}}{2})*2}{2*2}}$ $=\sqrt{\frac{2+\sqrt{3}}{4}}$ $=\frac{\sqrt{2+\sqrt{3}}}{\sqrt{4}}$ $=\frac{1}{2}\sqrt{2+\sqrt{3}}$
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