Which set of polar coordinates names the same point as [tex]\left(5, \frac{\pi}{4}\right)[/tex]?

A. [tex]\left(5, \frac{3 \pi}{4}\right)[/tex]
B. [tex]\left(5,-\frac{\pi}{4}\right)[/tex]
C. [tex]\left(-5, \frac{5 \pi}{4}\right)[/tex]
D. [tex]\left(-5, \frac{7 \pi}{4}\right)[/tex]



Answer :

Let's determine which set of polar coordinates corresponds to the same point as the given point [tex]\(\left(5, \frac{\pi}{4}\right)\)[/tex].

First, convert the given polar coordinates [tex]\(\left(5, \frac{\pi}{4}\right)\)[/tex] to Cartesian coordinates to understand the point in the [tex]\(xy\)[/tex]-plane:

[tex]\[ x = r \cos(\theta) = 5 \cos\left(\frac{\pi}{4}\right) = 5 \cdot \frac{\sqrt{2}}{2} = \frac{5\sqrt{2}}{2} \][/tex]
[tex]\[ y = r \sin(\theta) = 5 \sin\left(\frac{\pi}{4}\right) = 5 \cdot \frac{\sqrt{2}}{2} = \frac{5\sqrt{2}}{2} \][/tex]

So the Cartesian coordinates for the given polar coordinates [tex]\(\left(5, \frac{\pi}{4}\right)\)[/tex] are [tex]\(\left(\frac{5\sqrt{2}}{2}, \frac{5\sqrt{2}}{2}\right)\)[/tex].

Now, let's check each option by converting them to Cartesian coordinates and see which one matches [tex]\(\left(\frac{5\sqrt{2}}{2}, \frac{5\sqrt{2}}{2}\right)\)[/tex]:

### Option A: [tex]\(\left(5, \frac{3\pi}{4}\right)\)[/tex]

[tex]\[ x = 5 \cos\left(\frac{3\pi}{4}\right) = 5 \cdot \left(-\frac{\sqrt{2}}{2}\right) = -\frac{5\sqrt{2}}{2} \][/tex]
[tex]\[ y = 5 \sin\left(\frac{3\pi}{4}\right) = 5 \cdot \frac{\sqrt{2}}{2} = \frac{5\sqrt{2}}{2} \][/tex]

This gives Cartesian coordinates [tex]\(\left(-\frac{5\sqrt{2}}{2}, \frac{5\sqrt{2}}{2}\right)\)[/tex], which does not match [tex]\(\left(\frac{5\sqrt{2}}{2}, \frac{5\sqrt{2}}{2}\right)\)[/tex].

### Option B: [tex]\(\left(5, -\frac{\pi}{4}\right)\)[/tex]

[tex]\[ x = 5 \cos\left(-\frac{\pi}{4}\right) = 5 \cdot \frac{\sqrt{2}}{2} = \frac{5\sqrt{2}}{2} \][/tex]
[tex]\[ y = 5 \sin\left(-\frac{\pi}{4}\right) = 5 \cdot \left(-\frac{\sqrt{2}}{2}\right) = -\frac{5\sqrt{2}}{2} \][/tex]

This gives Cartesian coordinates [tex]\(\left(\frac{5\sqrt{2}}{2}, -\frac{5\sqrt{2}}{2}\right)\)[/tex], which does not match [tex]\(\left(\frac{5\sqrt{2}}{2}, \frac{5\sqrt{2}}{2}\right)\)[/tex].

### Option C: [tex]\(\left(-5, \frac{5\pi}{4}\right)\)[/tex]

[tex]\[ x = -5 \cos\left(\frac{5\pi}{4}\right) = -5 \cdot \left(-\frac{\sqrt{2}}{2}\right) = \frac{5\sqrt{2}}{2} \][/tex]
[tex]\[ y = -5 \sin\left(\frac{5\pi}{4}\right) = -5 \cdot \left(-\frac{\sqrt{2}}{2}\right) = \frac{5\sqrt{2}}{2} \][/tex]

This gives Cartesian coordinates [tex]\(\left(\frac{5\sqrt{2}}{2}, \frac{5\sqrt{2}}{2}\right)\)[/tex], which matches [tex]\(\left(\frac{5\sqrt{2}}{2}, \frac{5\sqrt{2}}{2}\right)\)[/tex].

### Option D: [tex]\(\left(-5, \frac{7\pi}{4}\right)\)[/tex]

[tex]\[ x = -5 \cos\left(\frac{7\pi}{4}\right) = -5 \cdot \frac{\sqrt{2}}{2} = -\frac{5\sqrt{2}}{2} \][/tex]
[tex]\[ y = -5 \sin\left(\frac{7\pi}{4}\right) = -5 \cdot \left(-\frac{\sqrt{2}}{2}\right) = \frac{5\sqrt{2}}{2} \][/tex]

This gives Cartesian coordinates [tex]\(\left(-\frac{5\sqrt{2}}{2}, \frac{5\sqrt{2}}{2}\right)\)[/tex], which does not match [tex]\(\left(\frac{5\sqrt{2}}{2}, \frac{5\sqrt{2}}{2}\right)\)[/tex].

Comparing all results, we see that only Option C [tex]\(\left(-5, \frac{5\pi}{4}\right)\)[/tex] gives the same point as [tex]\(\left(5, \frac{\pi}{4}\right)\)[/tex] in Cartesian coordinates.

Therefore, the correct option is [tex]\(\boxed{C}\)[/tex].

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