Maths Olympiad Prep

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Geometry Difficulty 5.6 AIME, harder Find the answer Italy

Problem:

Given a triangle ABCABC, let AA' be the reflection of AA with respect to CC, AA'' the reflection of AA with respect to BB, BB' the reflection of BB with respect to AA, BB'' the reflection of BB with respect to CC, CC' the reflection of CC with respect to BB and CC'' the reflection of CC with respect to AA. Determine the ratio between the area of ABCA'B'C' and that of the hexagon AACCBBA'A''C'C''B'B''.

Pick one

Solution

Solution:

The answer is (B). The segment ABA'B'' is the reflection of the segment ABAB with respect to the point CC. Therefore ABA'B'' and ABAB are parallel and congruent. Moreover AB=ABAB' = AB because BB' is the reflection of BB with respect to AA. Therefore the quadrilateral ABBAAB'B''A' is a parallelogram because it has a pair of opposite sides that are parallel and congruent.

Let CHCH be an altitude of ABCABC and AHA'H' an altitude of ABBAAB'B''A'. The triangles CAHCAH and AAHA'AH' are right triangles and have the angle CAH^\widehat{CAH} in common, so they are similar. Then, since AA=2ACAA' = 2AC, we have AH=2CHA'H' = 2CH. Therefore S(ABBA)=ABAH=AB2CH=4(12ABCH)=4S(ABC)S(AB'B''A') = AB' \cdot A'H' = AB \cdot 2CH = 4\left(\frac{1}{2} AB \cdot CH\right) = 4S(ABC).

Moreover S(ABA)=12S(ABBA)=2S(ABC)S(AB'A') = \frac{1}{2} S(AB'B''A') = 2S(ABC). Similarly, S(CAAC)=4S(ABC)S(CA'A''C') = 4S(ABC), S(CAC)=2S(ABC)S(CA'C') = 2S(ABC), S(BCCB)=4S(ABC)S(BC' C'' B') = 4S(ABC) and S(BCB)=2S(ABC)S(BC'B') = 2S(ABC).

Then S(ABC)=S(ABA)+S(CAC)+S(BCB)+S(ABC)=7S(ABC)S(A'B'C') = S(AB'A') + S(CA'C') + S(BC'B') + S(ABC) = 7S(ABC),
while S(AACCBB)=S(ABBA)+S(CAAC)+S(BCCB)+S(ABC)=13S(ABC)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(ABC)/S(AACCBB)=7/13S(A'B'C') / S(A'A''C'C''B'B'') = 7 / 13.

Figure 1

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Source: MathNet, licensed CC-BY-4.0. Statement translated into English from it; metadata (topic, difficulty) added by this project.