GeometryDifficulty 5.6AIME, harderFind the answerItaly
Problem:
Given a triangle ABC, let A′ be the reflection of A with respect to C, A′′ the reflection of A with respect to B, B′ the reflection of B with respect to A, B′′ the reflection of B with respect to C, C′ the reflection of C with respect to B and C′′ the reflection of C with respect to A. Determine the ratio between the area of A′B′C′ and that of the hexagon A′A′′C′C′′B′B′′.
Pick one
Solution
Solution:
The answer is (B). The segment A′B′′ is the reflection of the segment AB with respect to the point C. Therefore A′B′′ and AB are parallel and congruent. Moreover AB′=AB because B′ is the reflection of B with respect to A. Therefore the quadrilateral AB′B′′A′ is a parallelogram because it has a pair of opposite sides that are parallel and congruent.
Let CH be an altitude of ABC and A′H′ an altitude of AB′B′′A′. The triangles CAH and A′AH′ are right triangles and have the angle CAH in common, so they are similar. Then, since AA′=2AC, we have A′H′=2CH. Therefore S(AB′B′′A′)=AB′⋅A′H′=AB⋅2CH=4(21AB⋅CH)=4S(ABC).
Moreover S(AB′A′)=21S(AB′B′′A′)=2S(ABC). Similarly, S(CA′A′′C′)=4S(ABC), S(CA′C′)=2S(ABC), S(BC′C′′B′)=4S(ABC) and S(BC′B′)=2S(ABC).
Then S(A′B′C′)=S(AB′A′)+S(CA′C′)+S(BC′B′)+S(ABC)=7S(ABC), while S(A′A′′C′C′′B′B′′)=S(AB′B′′A′)+S(CA′A′′C′)+S(BC′C′′B′)+S(ABC)=13S(ABC).
Therefore S(A′B′C′)/S(A′A′′C′C′′B′B′′)=7/13.
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