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Geometry Difficulty 6.9 National olympiad Find the answer

Let ABC\triangle ABC be an equilateral triangle of side length 1. Let D,E,FD,E,F be points on BC,AC,ABBC,AC,AB respectively, such that DE20=EF22=FD38\frac{DE}{20} = \frac{EF}{22} = \frac{FD}{38}. Let X,Y,ZX,Y,Z be on lines BC,CA,ABBC,CA,AB respectively, such that XYDE,YZEF,ZXFDXY\perp DE, YZ\perp EF, ZX\perp FD. Find all possible values of 1[DEF]+1[XYZ]\frac{1}{[DEF]} + \frac{1}{[XYZ]}.

A number or a short expression. Spacing and $ signs are ignored.

Solution

Let ABC\triangle ABC be an equilateral triangle of side length 1. Let D,E,FD, E, F be points on BC,AC,ABBC, AC, AB respectively, such that DE20=EF22=FD38\frac{DE}{20} = \frac{EF}{22} = \frac{FD}{38}. Let X,Y,ZX, Y, Z be on lines BC,CA,ABBC, CA, AB respectively, such that XYDEXY \perp DE, YZEFYZ \perp EF, ZXFDZX \perp FD. We aim to find all possible values of 1[DEF]+1[XYZ]\frac{1}{[DEF]} + \frac{1}{[XYZ]}.

Consider the center KK of the spiral similarity Φ:DEFXYZ\Phi: \triangle DEF \to \triangle XYZ. By angle chasing, we conclude that KK is the Miquel point of D,E,FD, E, F with respect to ABC\triangle ABC. The transformation Φ\Phi rotates the plane by 9090^\circ, making KFKZKF \perp KZ, etc.

Let KPBCKP \perp BC, KQCAKQ \perp CA, KRABKR \perp AB, and θ:=KFB=KDC=KEA\theta := \angle KFB = \angle KDC = \angle KEA. We have:
1[DEF]+1[XYZ]=1[DEF](1+(KFKZ)2)=1[DEF](1+cot2θ)=1[DEF]sin2θ. \frac{1}{[DEF]} + \frac{1}{[XYZ]} = \frac{1}{[DEF]} \left(1 + \left(\frac{KF}{KZ}\right)^2\right) = \frac{1}{[DEF]} \left(1 + \cot^2 \theta\right) = \frac{1}{[DEF] \sin^2 \theta}.

Since KEFKQR\triangle KEF \sim \triangle KQR with ratio 1:sinθ1 : \sin \theta, we get [KEF]sin2θ=[KQR][KEF] \sin^2 \theta = [KQR]. Adding up, we find:
1[DEF]+1[XYZ]=1[PQR]. \frac{1}{[DEF]} + \frac{1}{[XYZ]} = \frac{1}{[PQR]}.

Now, QR=32AKQR = \frac{\sqrt{3}}{2} AK but QR=EFsinθQR = EF \sin \theta, so CK:AK:BK=DE:EF:FD=10:11:19CK : AK : BK = DE : EF : FD = 10 : 11 : 19. Let CK=10tCK = 10t.

To find tt, we use the fact that AC=291+606tAC = \sqrt{291 + 60 \sqrt{6}} t. Since AC=1AC = 1, we have t=1291+606t = \frac{1}{\sqrt{291 + 60 \sqrt{6}}}.

Next, we calculate [PQR][PQR]:
[PQR]=12PRQRsin(QRK+KRP)=12PRQRsin(KBA+KCA)=1232BK32CKsin(BKC60)=38(12BKCKsinBKC32BKCKcosBKC)=38[BKC]3332(BK2+CK2BC2). \begin{align*} [PQR] &= \frac{1}{2} PR \cdot QR \sin(\angle QRK + \angle KRP) \\ &= \frac{1}{2} PR \cdot QR \sin(\angle KBA + \angle KCA) \\ &= \frac{1}{2} \frac{\sqrt{3}}{2} BK \cdot \frac{\sqrt{3}}{2} CK \sin(\angle BKC - 60^\circ) \\ &= \frac{3}{8} \left( \frac{1}{2} BK \cdot CK \sin \angle BKC - \frac{\sqrt{3}}{2} BK \cdot CK \cos \angle BKC \right) \\ &= \frac{3}{8} [BKC] - \frac{3 \sqrt{3}}{32} (BK^2 + CK^2 - BC^2). \end{align*}

By rotational symmetry, we have similar expressions for the other segments. Adding them up, we get:
[PQR]=18[ABC]332(2AK2+2BK2+2CK2AB2BC2CA2)=332+33323321164t2=152194+406. \begin{align*} [PQR] &= \frac{1}{8} [ABC] - \frac{\sqrt{3}}{32} (2AK^2 + 2BK^2 + 2CK^2 - AB^2 - BC^2 - CA^2) \\ &= \frac{\sqrt{3}}{32} + \frac{3 \sqrt{3}}{32} - \frac{\sqrt{3}}{32} \cdot 1164 t^2 \\ &= \frac{15 \sqrt{2}}{194 + 40 \sqrt{6}}. \end{align*}

Thus, the value is:
972+40315. \boxed{\frac{97 \sqrt{2} + 40 \sqrt{3}}{15}}.

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.