Conversion between Cartesian and Polar Coordinates in Plane/Examples/(-2, -pi over 4)
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Example of Use of Conversion between Cartesian and Polar Coordinates in Plane
The point $P$ defined in polar coordinates as:
- $P = \polar {-2, -\dfrac \pi 4}$
can be expressed in the corresponding Cartesian coordinates as:
- $P = \tuple {\sqrt 2, -\sqrt 2}$
Proof
Let $P$ be expressed in Cartesian coordinates as:
- $P = \tuple {x, y}$
From Conversion between Cartesian and Polar Coordinates in Plane:
\(\ds x\) | \(=\) | \(\ds r \cos \theta\) | ||||||||||||
\(\ds y\) | \(=\) | \(\ds r \sin \theta\) |
where in this case:
\(\ds r\) | \(=\) | \(\ds -2\) | ||||||||||||
\(\ds \theta\) | \(=\) | \(\ds -\dfrac \pi 4\) |
Hence:
\(\ds x\) | \(=\) | \(\ds -2 \map \cos {-\dfrac \pi 4}\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds -2 \times \dfrac {\sqrt 2} 2\) | Cosine of $315 \degrees$ | |||||||||||
\(\ds \) | \(=\) | \(\ds \sqrt 2\) |
and:
\(\ds y\) | \(=\) | \(\ds -2 \map \sin {-\dfrac \pi 4}\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds -2 \times \paren {-\dfrac {\sqrt 2} 2}\) | Sine of $315 \degrees$ | |||||||||||
\(\ds \) | \(=\) | \(\ds -\sqrt 2\) |
Hence the result.
$\blacksquare$
Sources
- 1933: D.M.Y. Sommerville: Analytical Conics (3rd ed.) ... (previous) ... (next): Chapter $\text I$. Coordinates: $5$. Distance between two points in polar coordinates: Examples: $1 \ \text {(ii)}$