Precalculus (6th Edition) Blitzer

Published by Pearson
ISBN 10: 0-13446-914-3
ISBN 13: 978-0-13446-914-0

Chapter 5 - Section 5.4 - Product-to-Sum and Sum-to-Product Formulas - Exercise Set - Page 691: 55

Answer

The given statement makes sense.

Work Step by Step

One of the sum-to-product formulas is $\cos \alpha +\cos \beta =2\cos \frac{\alpha +\beta }{2}\cos \frac{\alpha -\beta }{2}$. So in this question, according to the above-mentioned formula, the value of $\alpha $ is ${{47}^{\circ }}$ and the value of $\beta $ is ${{59}^{\circ }}$. Now, the expression can be evaluated as provided below: $\begin{align} & \cos {{47}^{\circ }}+\cos {{59}^{\circ }}=2\cos \left( \frac{{{47}^{\circ }}+{{59}^{\circ }}}{2} \right)\cos \left( \frac{{{47}^{\circ }}-{{59}^{\circ }}}{2} \right) \\ & =2\cos \left( \frac{{{106}^{\circ }}}{2} \right)\cos \left( \frac{-{{12}^{\circ }}}{2} \right) \\ & =2\cos {{53}^{\circ }}\cos \left( -{{6}^{\circ }} \right) \end{align}$ Now, applying the even-odd identity, which is $cos(-x)=\cos x$, the expression can be further evaluated as given below: $2\cos {{53}^{\circ }}\cos \left( -{{6}^{\circ }} \right)=2\cos {{53}^{\circ }}\cos {{6}^{\circ }}$ Thus, $cos{{47}^{\circ }}+cos{{59}^{\circ }}$ can be expressed as $2\cos {{53}^{\circ }}\cos {{6}^{\circ }}$.
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