GeometryDifficulty 6.9National olympiadFind the answer
Let △ABC be an equilateral triangle of side length 1. Let D,E,F be points on BC,AC,AB respectively, such that 20DE=22EF=38FD. Let X,Y,Z be on lines BC,CA,AB respectively, such that XY⊥DE,YZ⊥EF,ZX⊥FD. Find all possible values of [DEF]1+[XYZ]1.
A number or a short expression. Spacing and $ signs are ignored.
Solution
Let △ABC be an equilateral triangle of side length 1. Let D,E,F be points on BC,AC,AB respectively, such that 20DE=22EF=38FD. Let X,Y,Z be on lines BC,CA,AB respectively, such that XY⊥DE, YZ⊥EF, ZX⊥FD. We aim to find all possible values of [DEF]1+[XYZ]1.
Consider the center K of the spiral similarity Φ:△DEF→△XYZ. By angle chasing, we conclude that K is the Miquel point of D,E,F with respect to △ABC. The transformation Φ rotates the plane by 90∘, making KF⊥KZ, etc.
Let KP⊥BC, KQ⊥CA, KR⊥AB, and θ:=∠KFB=∠KDC=∠KEA. We have: [DEF]1+[XYZ]1=[DEF]1(1+(KZKF)2)=[DEF]1(1+cot2θ)=[DEF]sin2θ1.
Since △KEF∼△KQR with ratio 1:sinθ, we get [KEF]sin2θ=[KQR]. Adding up, we find: [DEF]1+[XYZ]1=[PQR]1.
Now, QR=23AK but QR=EFsinθ, so CK:AK:BK=DE:EF:FD=10:11:19. Let CK=10t.
To find t, we use the fact that AC=291+606t. Since AC=1, we have t=291+6061.
Next, we calculate [PQR]: [PQR]=21PR⋅QRsin(∠QRK+∠KRP)=21PR⋅QRsin(∠KBA+∠KCA)=2123BK⋅23CKsin(∠BKC−60∘)=83(21BK⋅CKsin∠BKC−23BK⋅CKcos∠BKC)=83[BKC]−3233(BK2+CK2−BC2).
By rotational symmetry, we have similar expressions for the other segments. Adding them up, we get: [PQR]=81[ABC]−323(2AK2+2BK2+2CK2−AB2−BC2−CA2)=323+3233−323⋅1164t2=194+406152.
Thus, the value is: 15972+403.
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