Trigonometric Equations Question 346
Question: The solution set of $ (5+4\cos \theta )(2\cos \theta +1)=0 $ in the interval $
[0,2\pi ] $ is [EAMCET 2003]
Options:
A) $ { \frac{\pi }{3},,\frac{2\pi }{3} } $
B) $ { \frac{\pi }{3},,\pi } $
C) $ { \frac{2\pi }{3},\frac{4\pi }{3} } $
D) $ { \frac{2\pi }{3},\frac{5\pi }{3} } $
Show Answer
Answer:
Correct Answer: C
Solution:
- $ (5+4\cos \theta )(2\cos \theta +1)=0 $ $ \cos \theta =-5/4 $ , which is not possible.
$ \therefore 2\cos \theta +1=0 $ or $ \cos \theta =-1/2 $
Þ $ \theta =\frac{2\pi }{3},\frac{4\pi }{3}. $ Solution set is $ { \frac{2\pi }{3},\frac{4\pi }{3} }\in [0,2\pi ] $ .
BETA
