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Find the value of
[tex]\[ \cos \left(\frac{5 \pi}{3}\right) \][/tex]

A. [tex]\(\frac{\sqrt{3}}{2}\)[/tex]

B. [tex]\(-\frac{\sqrt{2}}{2}\)[/tex]

C. [tex]\(\frac{1}{2}\)[/tex]

D. [tex]\(\frac{\sqrt{2}}{2}\)[/tex]


Sagot :

Sure! Let's analyze the given trigonometric function step by step:

We need to find the value of [tex]\( \cos \left( \frac{5\pi}{3} \right) \)[/tex].

1. Determine the Quadrant:
[tex]\(\frac{5\pi}{3}\)[/tex] radians is an angle measured in the standard position. Since [tex]\(2\pi\)[/tex] radians represent a full circle (360 degrees), and [tex]\(\pi\)[/tex] represents half of a circle (180 degrees), we can see that:

[tex]\[ \frac{5\pi}{3} \text{ radians} = 2\pi - \frac{\pi}{3} \][/tex]

This tells us that [tex]\(\frac{5\pi}{3}\)[/tex] radians is equivalent to [tex]\(2\pi - \frac{\pi}{3}\)[/tex], which places the angle in the fourth quadrant.

2. Reference Angle:
The reference angle for [tex]\(\frac{5\pi}{3}\)[/tex] in the fourth quadrant is:

[tex]\[ 2\pi - \frac{5\pi}{3} = \frac{6\pi}{3} - \frac{5\pi}{3} = \frac{\pi}{3} \][/tex]

3. Cosine Value in the Fourth Quadrant:
In the fourth quadrant, the cosine function is positive. For the reference angle [tex]\(\frac{\pi}{3}\)[/tex]:

[tex]\[ \cos \left( \frac{\pi}{3} \right) = \frac{1}{2} \][/tex]

Therefore:

[tex]\[ \cos \left( \frac{5\pi}{3} \right) = \cos \left( 2\pi - \frac{\pi}{3} \right) = \cos \left( -\frac{\pi}{3} \right) = \frac{1}{2} \][/tex]

Given the choices:
A. [tex]\(\frac{\sqrt{3}}{2}\)[/tex]
B. [tex]\(-\frac{\sqrt{2}}{2}\)[/tex]
C. [tex]\(\frac{1}{2}\)[/tex]
D. [tex]\(\frac{\sqrt{2}}{2}\)[/tex]

The correct answer is:
[tex]\[ \boxed{\frac{1}{2}} \][/tex]
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