This figure depicts a regular pentagon inscribed in an equilateral triangle. Which color represents the greater area? Inspiration: here
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Let's assume the pentagon has side length 1.
Solve blue △ B G C . Its angles are easily deduced. ∠ G = 6 0 ∘ and ∠ C = 4 8 ∘
By the law of sines, B G = sin 6 0 ∘ sin 4 8 ∘ ≈ . 8 5 8
Since △ F A E and △ B G C are congruent, F A = B G and F G = 2 × B G + 1 ≈ 2 . 7 1 6
So △ F G H = 4 3 × F G 2 ≈ 3 . 1 9 5
The red area of the pentagon is 4 5 × 1 + 5 2 × A B 2 ≈ 1 . 7 2
Thus the total blue area is 3 . 1 9 5 − 1 . 7 2 ≈ 1 . 4 7 5 < 1 . 7 2
Red wins.