Calculus 8th Edition

Published by Cengage
ISBN 10: 1285740629
ISBN 13: 978-1-28574-062-1

Appendix D - Trigonometry - D Exercises - Page A33: 72


$0,\frac{\pi}{3},\frac{5 \pi}{3}, 2 \pi$

Work Step by Step

Need to find the range for $x$ for the equation $2+cos2x=3cosx$ $2+(2cos^{2}x-1)=3cosx$ $2cos^{2}x-3cosx+1=0$ $(2cosx-1)$ $(cosx-1)=0$ Here, $2cosx-1=0$ gives $cosx=\frac{1}{2}$ Since, $cosx>0$ on first and fourth quadrant thus, x must lie on $\frac{\pi}{3},\frac{5 \pi}{3}$ and $(cosx-1)=0$ gives $cosx=1$ $x =0, 2\pi$ Hence, $0,\frac{\pi}{3},\frac{5 \pi}{3}, 2 \pi$
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