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Sagot :
Certainly! Let's determine the value of [tex]\(\cos\left(\frac{3\pi}{4}\right)\)[/tex].
### Step-by-Step Solution:
1. Understand the Angle:
- The angle [tex]\(\frac{3\pi}{4}\)[/tex] is in radians.
- Converting this to degrees, we get:
[tex]\[ \frac{3\pi}{4} \times \frac{180^\circ}{\pi} = 135^\circ \][/tex]
2. Determine the Quadrant:
- The angle [tex]\(135^\circ\)[/tex] is in the second quadrant.
- Recall that in the second quadrant, the cosine function is negative.
3. Reference Angle:
- The reference angle for [tex]\(135^\circ\)[/tex] is:
[tex]\[ 180^\circ - 135^\circ = 45^\circ \][/tex]
4. Cosine of the Reference Angle:
- The cosine of [tex]\(45^\circ\)[/tex] (or [tex]\(\frac{\pi}{4}\)[/tex] radians) is:
[tex]\[ \cos 45^\circ = \frac{\sqrt{2}}{2} \][/tex]
5. Applying the Sign:
- Since [tex]\(135^\circ\)[/tex] is in the second quadrant, where cosine values are negative, we have:
[tex]\[ \cos 135^\circ = -\cos 45^\circ = -\frac{\sqrt{2}}{2} \][/tex]
Hence, the value of [tex]\(\cos\left(\frac{3\pi}{4}\right)\)[/tex] is:
[tex]\[ -\frac{\sqrt{2}}{2} \][/tex]
### Final Answer:
D. [tex]\(-\frac{\sqrt{2}}{2}\)[/tex]
### Step-by-Step Solution:
1. Understand the Angle:
- The angle [tex]\(\frac{3\pi}{4}\)[/tex] is in radians.
- Converting this to degrees, we get:
[tex]\[ \frac{3\pi}{4} \times \frac{180^\circ}{\pi} = 135^\circ \][/tex]
2. Determine the Quadrant:
- The angle [tex]\(135^\circ\)[/tex] is in the second quadrant.
- Recall that in the second quadrant, the cosine function is negative.
3. Reference Angle:
- The reference angle for [tex]\(135^\circ\)[/tex] is:
[tex]\[ 180^\circ - 135^\circ = 45^\circ \][/tex]
4. Cosine of the Reference Angle:
- The cosine of [tex]\(45^\circ\)[/tex] (or [tex]\(\frac{\pi}{4}\)[/tex] radians) is:
[tex]\[ \cos 45^\circ = \frac{\sqrt{2}}{2} \][/tex]
5. Applying the Sign:
- Since [tex]\(135^\circ\)[/tex] is in the second quadrant, where cosine values are negative, we have:
[tex]\[ \cos 135^\circ = -\cos 45^\circ = -\frac{\sqrt{2}}{2} \][/tex]
Hence, the value of [tex]\(\cos\left(\frac{3\pi}{4}\right)\)[/tex] is:
[tex]\[ -\frac{\sqrt{2}}{2} \][/tex]
### Final Answer:
D. [tex]\(-\frac{\sqrt{2}}{2}\)[/tex]
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