GeometryDifficulty 5.5AIME, harderFind the answerItaly
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). Let ABC be the triangle forming the base; the safety zone is a triangle A′B′C′ (with A′ belonging to the bisector of the angle at A, and cyclically for the others) inside the triangle ABC. Letting H and K be the projections of A′ and B′ respectively onto the side AB, we have A′H=1 meter. Since the triangle A′AH is half of an equilateral triangle (the angles at A, A′, and H are respectively 30∘, 60∘, and 90∘), the side AH has length 2⋅1⋅23=3, from which A′B′=HK=AB−AH−BK=AB−2⋅AH=8−2⋅3 meters. Therefore the area of triangle A′B′C′ is given by
(8−2⋅3)2⋅43=(64+12−323)⋅43=193−24,
which is the required area (in square meters).
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Source: MathNet,
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