Definition:Polar of Point

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Definition

Let $\KK$ be a conic section embedded in a Euclidean plane.

Let $P$ be an arbitrary point in that plane.

Let a secant line pass through $P$ and intersect $\KK$ at $L$ and $M$.


The polar of $P$ with respect to $\KK$ is the straight line upon which the tangents to $\KK$ intersect.


Circle

Let $\CC$ be a circle whose radius is $r$ and whose center is at the origin of a Cartesian plane.

Let $P = \tuple {x_0, y_0}$ be an arbitrary point in the Cartesian plane.


The polar of $P$ with respect to $\CC$ is the straight line whose equation is given by:

$x x_0 + y y_0 = r^2$


Ellipse

Let $\EE$ be an ellipse embedded in a Cartesian plane in reduced form with the equation:

$\dfrac {x^2} {a^2} + \dfrac {y^2} {b^2} = 1$


Let $P = \tuple {x_0, y_0}$ be an arbitrary point in the Cartesian plane.


The polar of $P$ with respect to $\EE$ is the straight line whose equation is given by:

$\dfrac {x x_0} {a^2} + \dfrac {y y_0} {b^2} = 1$


Also see

  • Results about polars of points can be found here.