## 19751Re: Cubics

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• Jan 16, 2011
Here is an old unanswered Hyacinthos message:

APH:

Let P be a point on the plane of triangle ABC.
The parallels to AB, AC through P intersect BC at Ab, Ac resp.
Let A1,A2 be the traces of the circumcenter,incenter of PAbAc on AbAc.

Similarly we define the points B1,B2; C1,C2.

Which are the loci of P such that (1) A1B1C1 (2) A2B2C2 be in perspective with ABC?

---

I found that the loci are the following cubics:

(1) K044 - Euler Central Cubic
(2) K033 - Spieker Central Cubic

Changing circumcenter, incenter by an arbitrary point Q=(u:v:w), the locus is the cubic

(cyclic sum) (u(v+w)y z((u+v-w)y -(u-v+w)z))) = 0

Central cubic with center the complement of Q.

--- In Hyacinthos@yahoogroups.com, xpolakis@... wrote:
>
> Let P be a point on the plane of triangle ABC.
> The parallels to AB, AC through P intersect BC at Ab, Ac resp.
> Let A1,A2 be the traces of the circumcenter,incenter of PAbAc on AbAc.
> Similarly we define the points B1,B2; C1,C2.
>
> Which are the loci of P such that (1) A1B1C1 (2) A2B2C2 be in perspective
> with ABC?
>
> I found that the loci are these cubics:
>
> xcos(B+u) + zcosu ycos(C+v) + xcosv zcos(A+w) + ycosw
> ----------------- * ------------------ * ------------------ = 1
> xcos(C-u) + ycosu ycos(A-v) + zcosv zcos(B-w) + xcosw
>
> where u := B-C, v := C-A, w := A-B for the (1), and
> 2u := B-C, 2v := C-A, 2w := A-B for the (2).
>
> But maybe these are Darboux cubics of some triangles of the reference
> triangle ABC...
>
> Antreas
>
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