Algebra and Trigonometry 10th Edition

Published by Cengage Learning
ISBN 10: 9781337271172
ISBN 13: 978-1-33727-117-2

Chapter 7 - 7.4 - Sum and Difference Formulas - 7.4 Exercises - Page 538: 28



Work Step by Step

$cos(u+v)=cos~u~cos~v-sin~u~sin~v$ $cos~\frac{\pi}{7}~cos~\frac{\pi}{5}-sin~\frac{\pi}{7}~sin~\frac{\pi}{5}=cos(\frac{\pi}{7}+\frac{\pi}{5})=cos(\frac{\pi(5)}{7(5)}+\frac{\pi(7)}{5(7)})=cos(\frac{5\pi}{35}+\frac{7\pi}{35})=cos~\frac{12\pi}{35}$
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