No hope to reach the center

Geometry Level 5

The image shows a sequence of inscribed regular polygons and circles. Each step, the number of sides of the polygon is increased by 1. Let r n r_n be the radius of the innermost circle after n n steps and r 0 = 1 r_0=1 , the radius of the largest circle.

Find lim n r n \displaystyle \lim_{n \to \infty}r_n


The answer is 0.114942.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

2 solutions

Gabriel Chacón
Jan 6, 2019

From the figure, we can see how the radii of two consecutive circles relate: r n = r n 1 cos ( π N ) r_n=r_{n-1}\cdot \cos\left(\dfrac{\pi}{N}\right) .

Applying the recurrence from the start, with r 0 = 1 r_0=1 , we get:

r = N = 3 cos ( π N ) 0.114942 r_{\infty}=\displaystyle \prod_{N=3}^{\infty}\cos\left(\dfrac{\pi}{N}\right)\approx0.114942


I evaluated the infinite product using Wolfram Alpha .

Aww, I was almost there, but I just was not able to get the value of that infinite product in Wolfram. That website kept loading for too long on my mobile device. 😔

Roliver Romero - 2 years, 5 months ago
Aaghaz Mahajan
Jan 6, 2019

See this ............

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...