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 $\delta$ instead of $\Delta$.

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 $S$ was one of the intersections of the circle through $C$ and $H$ centered on $\overrightarrow{AB}$, and likewise for $T$. 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 $S, H, C$ resp. $T, H, C$ be $\omega_S$ resp. $\omega_T$. Let those circles' centers be $O_S$ resp. $O_T$ and let those circles' other intersections with $\overrightarrow{AB}$ resp. $\overrightarrow{AD}$ be $S'$ resp. $T'$.

Note that $\overrightarrow{AH}$ and $\overrightarrow{AC}$ are isogonals with respect to $\angle BAD$.

We want to prove that the perpendicular bisectors of $SH$ and $HT$ intersect on $AH$. Observe that $S'H$ and $T'H$ are respectively parallel to those perpendicular bisectors (look at homotheties centered at $S$ resp. $T$ with ratio 2) and they intersect at $H$, so if we can establish a homothety centered at $A$ that sends $S'$ to $O_S$ and $T'$ to $O_T$ we're done. This is true iff the subdiagrams $A + \omega_S$ and $A + \omega_T$ are similar.

Now, compose an inversion centered at $A$ with radius $\sqrt{AH \cdot AC}$ with a reflection about the angle bisector of $\angle BAD$ (it doesn't matter in what order, they commute); this sends $H$ and $C$ to each other, so it sends $\omega_S$ to $\omega_T$. This proves that the two subdiagrams are similar because the angle between the lines of tangency from $A$ 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 $B$ and $D$ at all or the relation they imply between $C$ and $H$, 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.

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