oh right IMO 2014
by math_explorer, Jul 9, 2014, 1:14 AM
Solving along at home:
1: Pretty much the same as Davi Medeiros (post 6), except I used
instead of
.
2: Pretty much the same as SCP (post 2), except I was too lazy to describe the lower-bound peaceful configuration with anything other than a diagram.
3: So I figured out that
was one of the intersections of the circle through
and
centered on
, and likewise for
. I then followed it up with an obscenely contrived sequence of homotheties, reflections, and inversions that I wouldn't commit to paper for a million dollars.
Fine, that's hyperbole, I'm going to do it now. It looks like this:
Let the aforementioned circles through
resp.
be
resp.
. Let those circles' centers be
resp.
and let those circles' other intersections with
resp.
be
resp.
.
Note that
and
are isogonals with respect to
.
We want to prove that the perpendicular bisectors of
and
intersect on
. Observe that
and
are respectively parallel to those perpendicular bisectors (look at homotheties centered at
resp.
with ratio 2) and they intersect at
, so if we can establish a homothety centered at
that sends
to
and
to
we're done. This is true iff the subdiagrams
and
are similar.
Now, compose an inversion centered at
with radius
with a reflection about the angle bisector of
(it doesn't matter in what order, they commute); this sends
and
to each other, so it sends
to
. This proves that the two subdiagrams are similar because the angle between the lines of tangency from
to the circles is preserved at each step.
Click for diagram, which is way too large, oops
What can I say? Once a transformation addict, always a transformation addict?
Amusingly, I didn't use the points
and
at all or the relation they imply between
and
, except for implicitly at the very end to prove that the ending condition is equivalent to the problem's condition. So it looks like I proved a generalization of the problem. (!) Or else this is a fakesolve. (!) I hope a reader who is better at geometry than I am can tell me which one it is.
1: Pretty much the same as Davi Medeiros (post 6), except I used


2: Pretty much the same as SCP (post 2), except I was too lazy to describe the lower-bound peaceful configuration with anything other than a diagram.
3: So I figured out that





Fine, that's hyperbole, I'm going to do it now. It looks like this:
Let the aforementioned circles through










Note that



We want to prove that the perpendicular bisectors of















Now, compose an inversion centered at








Click for diagram, which is way too large, oops
What can I say? Once a transformation addict, always a transformation addict?
Amusingly, I didn't use the points



