Star inside pentagon

Geometry Level 5

The picture above shows a regular pentagon A B C D E ABCDE , with area of T T . Five equilateral triangles of side length A B AB were constructed inside the pentagon, example the triangle A B F ABF . The overlapping regions were shaded, and its area is denoted as U U .

Find 1000 U T \displaystyle \left \lfloor \frac{1000U}{T} \right \rfloor .


The answer is 193.

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1 solution

Chan Lye Lee
Apr 19, 2016

Let the point O O be the centre of the pentagon, and the point G G be the intersection point of sides of triangle with base E A EA and B C BC respectively.

Note that F B G = 3 π 5 π 3 = π 15 = 1 2 \angle FBG = \frac{3\pi}{5} - \frac{\pi}{3} = \frac{\pi}{15}=12 ^{\circ} . This implies that G B A = π 3 π 15 = 4 π 15 = 4 8 \angle GBA = \frac{\pi}{3} - \frac{\pi}{15} = \frac{4\pi}{15}=48 ^{\circ} and that O B A = 4 π 15 + 1 2 × π 15 = 3 π 10 = 5 4 \angle OBA = \frac{4\pi}{15} + \frac{1}{2}\times \frac{\pi}{15}=\frac{3\pi}{10}=54 ^{\circ}

Now, the area of the triangle A G B AGB is A B 2 tan 4 8 4 \frac{AB^2\tan 48 ^{\circ}}{4} while the area of the triangle A O B AOB is A B 2 tan 5 4 4 \frac{AB^2\tan 54 ^{\circ}}{4}

Hence 1000 U T \displaystyle \left \lfloor \frac{1000U}{T} \right \rfloor = 1000 ( 1 tan 4 8 tan 5 4 ) = 193 =\left \lfloor 1000 \left(1-\frac{\tan 48 ^{\circ}}{\tan 54 ^{\circ}}\right) \right \rfloor=193

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