Five in Three

Geometry Level 3

This figure depicts a regular pentagon inscribed in an equilateral triangle. Which color represents the greater area? Inspiration: here

Red Blue Neither

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1 solution

Fletcher Mattox
Sep 27, 2020

Let's assume the pentagon has side length 1.

Solve blue B G C \triangle BGC . Its angles are easily deduced. G = 60 \hspace{.2cm}\angle{G} = {60}^\circ and C = 48 \angle{C} = {48}^\circ

By the law of sines, B G = sin 48 sin 60 . 858 \overline{BG} = \frac{\sin{48}^\circ}{\sin{60}^\circ} \approx .858

Since F A E \triangle{FAE} and B G C \triangle{BGC} are congruent, F A = B G \hspace{.2cm}\overline{FA} = \overline{BG} and F G = 2 × B G + 1 2.716 \hspace{.2cm}\overline{FG} = 2\times\overline{BG} + 1 \approx 2.716

So F G H = 3 4 × F G 2 3.195 \triangle{FGH} = \frac{\sqrt{3}}{4}\times\overline{FG}^2 \approx 3.195

The red area of the pentagon is 5 4 × \frac{5}{4}\times 1 + 2 5 \sqrt{ 1 + \frac{2}{\sqrt{5} }} × A B 2 1.72 \times \overline{AB}^2 \approx 1.72

Thus the total blue area is 3.195 1.72 1.475 < 1.72 3.195 - 1.72 \approx 1.475 < 1.72

Red wins.

It should read.

Since \triangle{FAE}△FAE and \triangle{BGC}△BGC are congruent, \hspace{.2cm}\overline{FA} = \overline{BG}

Vijay Simha - 8 months, 2 weeks ago

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Thank you. How were you able to view my LaTeX? I could improve my style if I could view others.

Fletcher Mattox - 8 months, 2 weeks ago

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