Answer
$cos\theta cos\phi-sin\theta sin\phi$
Work Step by Step
We know that $cos(A+B)=cosAcosB-sinAsinB$
Let $A=\theta$ and $B=\phi$
$\implies cos(\theta+\phi)=cos\theta cos\phi-sin\theta sin\phi$
Trigonometry 7th Edition
by McKeague, Charles P.; Turner, Mark D.
Published by
Cengage Learning
ISBN 10:
1111826854
ISBN 13:
978-1-11182-685-7
Chapter 5 - Section 5.2 - Sum and Difference Formulas - 5.2 Problem Set - Page 288: 3
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