Answer
$\cos(9x)$
Work Step by Step
We know that $\cos(A+B)=\cos A\cos B-\sin A\sin B$, thus $\cos 4x\cos 5x-\sin 4x\sin 5x=\cos(4x+5x)=\cos(9x)$
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 - Test - Page 315: 19
Update this answer!
You can help us out by revising, improving and updating this answer.
Update this answerAfter you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.
