Deriving the Polar of a Point on a Conic

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In summary, the polar of a point (p, q) with respect to a general conic is given by apx + h(py + qx) + bgy + g(p + x) + f(q + y) + c = 0. This is derived by considering the chord of contact of the tangents from P to the conic. If the point lies on the conic, the chord of contact would not exist. The equation of the tangent line from P is needed to derive the polar, which can be found using implicit differentiation.
  • #1
Appleton
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If a general conic is

[itex]
ax^2+2hxy+by^2+2gx+2fy+c=0
[/itex]

I am told that, if P(p, q) is a point on this conic, then the polar of P(p, q) to this conic is

[itex]
apx+h(py+qx)+bgy+g(p+x)+f(q+y)+c=0
[/itex]

How is this derived?
 
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  • #2
What is your definition of the polar of a point wrt the conic? What do you know about the polar if the point lies on the conic?
 
  • #3
Thanks for your reply micromass. I realize I made a mistake. P(p,q) does not lie on the conic.

The polar is the chord of contact of the tangents from P.

If the point lies on the conic then the chord of contact would be non existent as P and the tangent points would all be coincident.

If we assume there are no constraints on P, what would be the derivation?
 
  • #4
Do you know the equation of the tangent line from ##P##?
 
  • #5
OK I think I'm with you now. Thanks for the prompt. I think the implicit derivative was my main stumbling block, amongst various other oversights.
 

What is the definition of "pole and polar of a conic"?

The pole and polar of a conic are two fundamental concepts in analytic geometry. The pole is a fixed point outside the conic, and the polar is a line passing through the pole and intersecting the conic at two points.

How are the pole and polar related to each other?

The pole and polar are dual concepts, meaning that they are related through a one-to-one correspondence. This means that every point on the polar corresponds to a unique line passing through the pole, and vice versa.

What is the significance of the pole and polar in conics?

The pole and polar are important tools for studying conic sections because they provide a geometric interpretation of the algebraic equations for conic sections. They allow us to easily determine properties such as tangents, normals, and points of intersection on a conic.

How can the pole and polar be used in real-life applications?

The concepts of pole and polar have various applications in fields such as engineering, astronomy, and physics. For example, in optics, the pole and polar of a parabola can be used to determine the focus and directrix of a parabolic mirror.

Can the pole and polar be generalized to higher dimensions?

Yes, the concept of pole and polar can be extended to higher dimensions in the study of quadrics, which are three-dimensional surfaces defined by second-degree equations. In this case, the pole becomes a fixed point in space, and the polar becomes a plane passing through the pole and intersecting the quadric at a conic section.

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