The picture above shows a regular pentagon , with area of . Five equilateral triangles of side length were constructed inside the pentagon, example the triangle . The overlapping regions were shaded, and its area is denoted as .
Find .
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Let the point O be the centre of the pentagon, and the point G be the intersection point of sides of triangle with base E A and B C respectively.
Note that ∠ F B G = 5 3 π − 3 π = 1 5 π = 1 2 ∘ . This implies that ∠ G B A = 3 π − 1 5 π = 1 5 4 π = 4 8 ∘ and that ∠ O B A = 1 5 4 π + 2 1 × 1 5 π = 1 0 3 π = 5 4 ∘
Now, the area of the triangle A G B is 4 A B 2 tan 4 8 ∘ while the area of the triangle A O B is 4 A B 2 tan 5 4 ∘
Hence ⌊ T 1 0 0 0 U ⌋ = ⌊ 1 0 0 0 ( 1 − tan 5 4 ∘ tan 4 8 ∘ ) ⌋ = 1 9 3