Answer
$$-\frac{\sqrt{6}-\sqrt{2}}{4}$$
Work Step by Step
Finding the exact value using the Laws of Trigonometry, we obtain:
$$\cos \left(90^{\circ \:}-345^{\circ \:}\right) \\ \cos \left(-255^{\circ \:}\right) \\ -\frac{\sqrt{6}-\sqrt{2}}{4}$$
Functions Modeling Change: A Preparation for Calculus, 5th Edition
by Connally, Eric; Hughes-Hallett, Deborah; Gleason, Andrew M.; Cheifetz, Phil C.
Published by
Wiley
ISBN 10:
1118583191
ISBN 13:
978-1-11858-319-7
Chapter 9 - Trigonometric Identities, Models, and Complex Numbers - 9.3 Sum and Difference Formulas for Sine and Cosine - Exercises and Problems for Section 9.3 - Exercises and Problems - Page 368: 10
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