Answer
$ -\frac{1}{4}$
Work Step by Step
$cos285^\circ\cdot cos195^\circ=\frac{1}{2}[cos(285^\circ+195^\circ)+cos(285^\circ-195^\circ)]=\frac{1}{2}[cos(480^\circ)+cos(90^\circ)]=\frac{1}{2}(cos(120^\circ)+0)=-\frac{1}{4}$
Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)
by Sullivan III, Michael
Published by
Pearson
ISBN 10:
0-32193-104-1
ISBN 13:
978-0-32193-104-7
Chapter 6 - Analytic Trigonometry - Section 6.7 Product-to-Sum and Sum-to-Product Formulas - 6.7 Assess Your Understanding - Page 524: 2
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