Maths Olympiad Prep

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Geometry Difficulty 5.5 AIME, harder Find the answer Italy

Problem:

The reds and the greens are having a water balloon fight. The base of the reds is an area in the shape of an equilateral triangle with side 8 meters. The greens cannot enter the base of the reds, but they can throw their projectiles into the base while still staying outside the perimeter. Knowing that the greens manage to hit a target up to a maximum distance of 1 meter, how large (in square meters) is the zone inside the base of the reds that is safe from the range of fire of the greens?

Pick one

Solution

Solution:

The answer is (A)\mathbf{(A)}. Let ABCABC be the triangle forming the base; the safety zone is a triangle ABCA'B'C' (with AA' belonging to the bisector of the angle at AA, and cyclically for the others) inside the triangle ABCABC. Letting HH and KK be the projections of AA' and BB' respectively onto the side ABAB, we have AH=1A'H=1 meter. Since the triangle AAHA'AH is half of an equilateral triangle (the angles at AA, AA', and HH are respectively 3030^\circ, 6060^\circ, and 9090^\circ), the side AHAH has length 2132=32 \cdot 1 \cdot \frac{\sqrt{3}}{2} = \sqrt{3}, from which AB=HK=ABAHBK=AB2AH=823A'B' = HK = AB - AH - BK = AB - 2 \cdot AH = 8 - 2 \cdot \sqrt{3} meters. Therefore the area of triangle ABCA'B'C' is given by

Figure 1

(823)234=(64+12323)34=19324,(8-2 \cdot \sqrt{3})^2 \cdot \frac{\sqrt{3}}{4} = (64 + 12 - 32 \sqrt{3}) \cdot \frac{\sqrt{3}}{4} = 19 \sqrt{3} - 24,

which is the required area (in square meters).

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