The Dumpty Parabola (So, sad to know :( That it's already discovered :( )
by AlastorMoody, Sep 11, 2019, 4:15 PM
This Parabola is already discovered
and is also known as Artzt Parabola, But anyway, I'll like to call this "Dumpty Parabola"
...
Let's try to motivate this parabola!
Well, *coincidence* I was also working on the same construction, 'coz It is almost impossible to construct a parabola tangent to two lines at given point on GEOGEBRA...
While searching for some Lemmas, in SL Loney's Coordinate Geometry book, I found the following Lemma:
Now, this Lemma clearly motivates the construction of a parabola with focus at Dumpty Point and so we have the following Problem
Problem: Let
be a triangle. Let
be the midpoint of
symmedian Chord. Let
be midpoint of
. Let
intersect Nine-Point Circle at
. Let
be perpendicular to
at
. Prove that, There Exists a Parabola,
with Focus at
tangent to
at
and Directrix as 
Proof: (By Supercali)
Proof: (By MathPassionForever)
This discussion could have ended here, but AoPS guys are just too amazing and they found many more properties which I couldn't have managed to find out nor could I have proved them!! >_< - https://artofproblemsolving.com/community/q1h1912112p13117117
Here's a Property that can be killed using the SL Loney Similarity Lemma
These are complete Supercali's efforts:
Proof: Let
be the midpoint of
. Using angle chasing, we can get
(you will have to use concyclicities used in my original solution). Let
meet
at
. Then, by angle chasing,
is a kite. Hence
and we are done.
These are math_pi_rate's efforts at providing such amazing proofs to these properties:
This is from MathPassionForever's Blog,
Such a Clean report, I guess I'll ever be able to present this so neatly 


Let's try to motivate this parabola!
uraharakisuke_hsgs wrote:
Let
be two abitrary lines that are not perpendicular . Let
be abitrary points lie on
. How to construct a parabola that is tangent to
at 






Similarity on Parabola Lemma wrote:
Let
be a Parabola with
. Let the tangents at
meet at
. Then, 





Problem: Let















Proof: (By Supercali)
Supercali wrote:
Let
and
be midpoints of
and
respectively. Let
meet
at
respectively. Let
meet
at
and
at
. By angle chasing (using property of symmedians, midpoint theorem and the fact that
are concyclic), we can easily see that
are concyclic. Hence
. Similarly
. Hence it will be sufficient to show that
lie on the parabola with focus
and directrix
.
Let
be foot from
on
.
. By angle chasing, we can easily see that
, hence
is a kite, which implies
, hence
lies on the parabola. Similarly,
also lies on the parabola, and we are done.



















Let









MathPassionForever wrote:
Consider an inparabola of
with directrix as perpendicular bisector of
. Note that since the foci of the parabola are isogonal conjugates, if the axis is parallel to the
median, the other focus is on the
symmedian. Also, since the isogonal conjugate of a point at infinity is on the circumcircle, the focus lies on
, which is just a homothety of ratio
of
from
. So clearly
lies on the former. So
is the focus of this inparabola!










This discussion could have ended here, but AoPS guys are just too amazing and they found many more properties which I couldn't have managed to find out nor could I have proved them!! >_< - https://artofproblemsolving.com/community/q1h1912112p13117117
Here's a Property that can be killed using the SL Loney Similarity Lemma
Aryan-23 wrote:
Prove that the circumcircle of the triangle formed by tangents at any 3 points on the parabola passes through its focus??
These are complete Supercali's efforts:
Supercali wrote:
Another interesting property:
is tangent to the parabola and point of intersection lies on
.
This gives another interesting problem:
In
, Let
be a point such that
is harmonic. Let
be the line passing through the orthocentre of the triangle which is perpendicular to the
-median. Prove that the parabola with focus
and directrix
is tangent to the sides of 


This gives another interesting problem:
In
















Supercali wrote:
The niceness continues:
1) Let
meet the parabola at
Then
passes through the centroid of 
2) Let
be the
-Humpty point and let
be the orthocentre. Then
, foot from
onto
,
defined above and midpoint of
are concyclic.
1) Let




2) Let








Supercali wrote:
Another thing:
Let
meet the parabola again at
. Then
is parallel to
.
Also, if
meets
at
, then
is the midpoint of
.
Let




Also, if





Supercali wrote:
Let us call it the Artzt parabola from now on. Here are some properties of its tangents and tangency points:
1) (Well known) Let
be any point on
and let
be points on
respectively such that
is a parallelogram. Then
is tangent to the parabola.
2) If
is the point of tangency, and
meets
at
, then
is the midpoint of
.
3) (Unsolved, but my favourite) Apparently
(midpoint of
),
(midpoint of
),
,
,
,
,
(intersection of
and
) lie on a conic.
4) (Unsolved) Apparently the locus of the centre of the above conic as
varies on
is a line passing through the midpoint of
(not shown in figure).
1) (Well known) Let






2) If






3) (Unsolved, but my favourite) Apparently











4) (Unsolved) Apparently the locus of the centre of the above conic as



These are math_pi_rate's efforts at providing such amazing proofs to these properties:
math_pi_rate wrote:
Here are (brief) proofs of some of the properties (Assuming that the properties in post #1 and post #3 are solved; Also, I'll use the diagram in post #10):
Let
denote the parabola from now on. Let
and
. It's easy to see that
is a parallelogram, which directly gives that
is also a parallelogram (i.e.
). By Pascal on
, we get that
lies on
, which gives
, or that
.
Now, Let
. It's easy to show that
and
are all parallelograms. This also implies that
and that
bisects
(i.e.
). Then, Pascal on
and
gives that
are collinear. As
, so we get
, as desired.
Note that
are concyclic. Then POP, and dividing both sides by half, easily gives that
are concyclic. Now,
By Pascal on
, it suffices to show that the line joining
and
is parallel to
. This is easy by some cross ratio chasing (I am seriously not in the mood of writing that up
).
Animate
on the parabola
. One can easily show that
are projective maps (Try to use the fact that focus of the the parabola lies on the circle passing through the point of intersection of the tangents at any two points, and the point where these tangents meet the tangent at the vertex). And,
and
are also projective. Thus it suffices to prove the problem for three positions of
. Take
for this.
(midpoint of
),
(midpoint of
),
,
,
,
,
(intersection of
and
) lie on a conic.
Apply Pascal on
and
to get that
are concurrent. Then, the converse of Pascal's Theorem on
and
gives the desired conclusion.
Supercali wrote:
Let
meet the parabola at
Then
passes through the centroid of 















Now, Let












Supercali wrote:
Let
be the
-Humpty point and let
be the orthocentre. Then
, foot from
onto
(say
),
defined above and midpoint of
(say
) are concyclic.













Supercali wrote:
Let
meet the parabola again at
. Then
is parallel to
.









Supercali wrote:
Let
be any point on
and let
be points on
respectively such that
is a parallelogram. Then
is tangent to the parabola at a point
.














Supercali wrote:
















This is from MathPassionForever's Blog,
Quote:
I am extremely indebted to AlastorMoody, Supercali and math_pi_rate for this one. I admittedly have just shifted an entire thread to my blog post, but this was just too beautiful so........
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So here is the description of some of the objects:
is the reference triangle
are the midpoints of sides 
is the midpoint of
and
is the midpoint of 
is the
-Humpty point
is the orthocenter of
,
is the foot of
-altitude,
is the orthocenter of
and hence the
midpoint of
.
is the
-Dumpty point
is the reflection of
about
and hence the midpoint of 
is the line through
perpendicular to 
are the intersections of
with
respectively
are the feet of perpendiculars from
on 
is the intersection of
and 
and
will be defined later.
are
and
respectively.
Here's the first part:
EXISTENCE OF ARTZT PARABOLA: Prove that there exists a parabola tangent to
at
respectively. Prove that its focus is
and its directrix is
. It will be denoted by
from now on.
My partial proof : Consider the inparabola of
with its axis parallel to the
-median. Then its focus must lie on the
-symmedian as well as on
and hence is
. Since its directrix is perpendicular to
and passes through
(Orthocenter always lies on directrix of inparabola), the directrix must be
. From here, I have only Supercali's proof for tangency to
. For tangency at
, note that
is the perpendicular bisector of
and that 
Supercali's pure angle chase proof:
(i)
are cyclic and so are 
Proof:
and we have
cyclic.
and we have
cyclic. Similarly for the other one.
(ii)
so it suffices to show
lie on the parabola.
(iii)
which makes
a kite
and we're done!
Corollary 1:
share an Artzt Parabola and hence a Dumpty point!
Proof 1: By degrees of freedom, a unique parabola is tangent to
at
. But since the parabola is tangent to
at
and
at
we're done!
And then math_pi_rate reminds us that Pascal might be a little more than a unit of pressure at times:
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real parabola1 (real x) {return x^2/2/3.5805800598820867;}
draw(shift((-0.09542226795749895,-0.4882896694895455))*rotate(-165.59193348613547)*graph(parabola1,-21.48348035929252,21.48348035929252), linewidth(0.5) + ffqqtt); /* parabola construction */
draw(circle((1.5587528028867694,-0.3267002463918187), 2.2481451186557893), linewidth(0.4) + qqccqq);
draw((-0.75982,4.94852)--(1.977960616629467,-5.708202649215887), linewidth(0.8) + dotted + wrwrwr);
draw((xmin, -0.3902484892484888*xmin-2.0856656982162205)--(xmax, -0.3902484892484888*xmax-2.0856656982162205), linewidth(0.8) + dotted + wrwrwr); /* line */
draw((xmin, 0.8073184282603932*xmin-2.504872661830268)--(xmax, 0.8073184282603932*xmax-2.504872661830268), linewidth(0.8) + dotted + wrwrwr); /* line */
draw(circle((-0.5826055744380011,-0.44652354909410774), 2.0057731677920785), linewidth(0.4) + qqccqq);
draw((-4.4106718182458415,-6.065689301708911)--(3.8033865257523876,-0.2010979983614316), linewidth(0.4) + linetype("4 4") + wrwrwr);
draw((8.366593051504776,-5.3507159967228635)--(-2.585245909122921,-0.5585846508544554), linewidth(0.4) + linetype("4 4") + wrwrwr);
draw((xmin, 1.6351611194359528*xmin + 1.1464697660785685)--(xmax, 1.6351611194359528*xmax + 1.1464697660785685), linewidth(0.4) + ffqqff); /* line */
draw((xmin, -0.8214199378593325*xmin + 1.5217703477385132)--(xmax, -0.8214199378593325*xmax + 1.5217703477385132), linewidth(0.4) + ffqqff); /* line */
fill(shift((-0.34509169785063354,-2.4630761093789717)) * scale(0.10583333333333333) * ((1,0)--(0,1)--(-1,0)--(0,-1)--cycle)); /* special point */
draw((xmin, 0.05595667870036119*xmin-0.41392287615624096)--(xmax, 0.05595667870036119*xmax-0.41392287615624096), linewidth(0.4) + wwccff); /* line *//* special point */
draw(circle((-4.066966825028381,-2.5001975640019505), 5.134319220144066), linewidth(0.6) + linetype("4 4") + ffwwqq);
draw(shift((-0.15682087388725796,-5.82765793117621)) * scale(0.17638888888888887) * ((0,1)--(0,-1)^^(1,0)--(-1,0))); /* special point */
/* dots and labels */
dot((-0.75982,4.94852),linewidth(3pt) + dotstyle);
label("$A$", (-0.6743597083187647,5.143247028072334), NE * labelscalefactor);
dot((-4.4106718182458415,-6.065689301708911),linewidth(3pt) + dotstyle);
label("$B$", (-4.318417198247642,-5.881180061712511), NE * labelscalefactor);
dot((8.366593051504776,-5.3507159967228635),linewidth(3pt) + dotstyle);
label("$C$", (8.458847671502976,-5.1662067567264645), NE * labelscalefactor);
dot((1.977960616629467,-5.708202649215887),linewidth(3pt) + dotstyle);
label("$M$", (2.070215236627668,-5.581352546718363), NE * labelscalefactor);
dot((3.8033865257523876,-0.2010979983614316),linewidth(2pt) + dotstyle);
label("$B'$", (3.892243981592106,-0.06913900182593982), SW * labelscalefactor);
dot((-2.585245909122921,-0.5585846508544554),linewidth(2pt) + dotstyle);
label("$C'$", (-2.496388453283203,-0.4150938268191881), NE * labelscalefactor);
dot((0.6090703083147335,-0.3798413246079435),linewidth(2pt) + dotstyle);
label("$Q$", (0.7094595916542265,-0.23058458682278904), NE * labelscalefactor);
dot((0.35004888452167676,-2.2222717465639237),linewidth(2pt) + dotstyle);
label("$D$", (0.43269573165962827,-2.07567698678678), N * labelscalefactor);
dot((0.1461169215727915,1.4221897848451754),linewidth(2pt) + dotstyle);
label("$M'$", (0.24818649166322942,1.568380503142102), NE * labelscalefactor);
dot((-5.933120871784682,-0.13960551962021883),linewidth(2pt) + dotstyle);
label("$D'_{1}$", (-5.840618428217931,0.000051963172709838995), NE * labelscalefactor);
dot((6.225354714930267,2.983985089310572),linewidth(2pt) + dotstyle);
label("$D'_{2}$", (6.313927756544841,3.1136453881119444), NE * labelscalefactor);
dot((1.1640079116974356,-2.539918027229433),linewidth(2pt) + dotstyle);
label("$R$", (1.262987311643423,-2.3985681567804784), SE * labelscalefactor);
dot((0.9566005160561046,-1.7325914367347726),linewidth(2pt) + dotstyle);
label("$P$", (1.0554144166474742,-1.5913402317962324), NE * labelscalefactor);
dot((-1.3683749134976222,-1.0910436892842783),linewidth(2pt) + dotstyle);
label("$S$", (-1.2740147383070608,-0.9455578918088355), NE * labelscalefactor);
dot((2.8907133764192507,-0.8527192542889295),linewidth(2pt) + dotstyle);
label("$T$", (2.992761436609662,-0.7149213418133367), SE * labelscalefactor);
dot((1.953497694076105,1.886517534445795),linewidth(2pt) + dotstyle);
label("$E$", (2.047151581628118,2.0296536031331), NE * labelscalefactor);
dot((-2.121811664932106,0.839544338546558),linewidth(2pt) + dotstyle);
label("$F$", (-2.035115353292206,0.9687254731538051), NE * labelscalefactor);
dot((1.052053843145583,-2.1041404303096494),linewidth(2pt) + dotstyle);
label("$X_{a}$", (1.1476690366456737,-1.9603587117890304), E * labelscalefactor);
label("$H$", (-0.2592139183268673,-2.3293771917818287), SW * labelscalefactor*2);
dot((-8.950368109446176,-0.9147557487064798),linewidth(2pt) + dotstyle);
label("$K$", (-8.861957233158963,-0.7841123068119863), NE * labelscalefactor); dot((-0.5524558489253169,1.2427219453105143),linewidth(1pt) + dotstyle + invisible,UnFill(0));
label("$H_{A}$", (-0.46678681332281596,1.4299985731448028), N * labelscalefactor);
label("$H_{a}$", (-0.0747046783304685,-5.604416201717912), N * labelscalefactor);
clip((xmin,ymin)--(xmin,ymax)--(xmax,ymax)--(xmax,ymin)--cycle);
[/asy]](http://latex.artofproblemsolving.com/d/9/3/d931a8e4e2e9408053038f49f5a14eab6cd73d6e.png)
So here is the description of some of the objects:















midpoint of
























Here's the first part:
EXISTENCE OF ARTZT PARABOLA: Prove that there exists a parabola tangent to





My partial proof : Consider the inparabola of













Supercali's pure angle chase proof:
(i)


Proof:




(ii)


(iii)



Corollary 1:

Proof 1: By degrees of freedom, a unique parabola is tangent to






And then math_pi_rate reminds us that Pascal might be a little more than a unit of pressure at times:

This post has been edited 9 times. Last edited by AlastorMoody, Oct 14, 2019, 6:50 AM