Indian Statistical Institute (ISI) B.Math & B.Stat : Trigonometry

Find the ratio of the areas of the regular pentagons inscribed and circumscribed around a given circle. Let $a$ be the side of the circumscribed pentagon and $b$ be that of the inscribed pentagon. First note that for the circle is inscribed for the exterior pentagon and circumscribed for the interior pentagon. Therefore the $in-radius$ of the exterior polygon, say $r$ is equal to the $circum-radius$, say $R$ of the interior pentagon, i.e., $R=r$. See the figure below. Using standard formula, $$a = 2 r \tan \frac{\pi}{5}, \quad b = 2 R \sin \frac{\pi}{5}$$ . Area of a regular polygon having $n$ sides is $n \times \frac {(side)^2}{4} \cot \frac{\pi}{n}$. Therefore the required ratio is $\bigg( \frac{5 \times \frac {b^2}{4} \cot \frac{\pi}{5}}{5 \times \frac {a^2}{4} \cot \frac{\pi}{5}} \bigg) = \frac{b^2}{a^2} = \frac{(2 R \sin \frac{\pi}{5})^2} {(2 r \tan \frac{\pi}{5})^2} = \cos^2 \frac{\pi}{5}$