Good n-gon Part II

Geometry Level 4

Two regular 2015-gons are circumscribed to and inscribed to a given circle. The angular point of one of them are joined to each of the angular points of the other.

Denote α \alpha as the sum of squares of the straight lines as drawn, and denote γ \gamma as the sum of area of the two polygons.

What is the value of α γ \frac { \alpha}{\gamma} ? Give your answer to 3 decimal places.

Check Out Good n-gon Part I .


The answer is 641.077.

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1 solution

This was a beautiful problem. Although my answer is not exactly same as the one given.

We will use a small concept in physics to solve this question: Moment of inertia.

Take the center of circle as origin O O . Now, we want to find out sum of squares when one vertex of smaller polygon is connected to all the vertices of bigger one. Assume mass of each vertex as unity.

Moment of inertia I I of larger polygon about O O :

I O = 2015 x 2 I_{O}=2015x^2 , where x x is distance of one vertex from O O .

Moment of inertia about one of the vertices of the smaller polygon is given by:

I v = I O + 2015 r 2 I_{v}=I_{O}+2015r^2 , where r r is radius of circle.

I v = \Rightarrow I_{v}= sum of squares of lengths = α = 2015 ( x 2 + r 2 ) =\alpha=2015(x^2+r^2)

Sum of areas of the polygons is simply: γ = π ( x 2 + r 2 ) \gamma=\pi(x^2+r^2) , since a 2015 2015 -gon can be approximated to a circle.

Hence: α / γ = 641.394 \alpha/\gamma=\boxed{641.394}

Brilliant accepted this approximate value.

@Vraj Mehta

This is a beautiful solution (as is the question) :)... The difference in your answer and the given answer obviously arises because of your approximation of a polygon as a circle.

Shashwat Shukla - 6 years, 3 months ago

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It doesn't. Check. The answer remains same to the 7th decimal place or something.

Raghav Vaidyanathan - 6 years, 3 months ago

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Yea...So it does...My apologies...Looks like your answer is correct then...

Vraj Mehta :Can you please explain?

Shashwat Shukla - 6 years, 3 months ago

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@Shashwat Shukla The answer is 2 sin 2 π n \dfrac{2}{\sin\frac{2\pi}{n}} ,where n n is the number of sides.

And Since The Number is Large Enough( n = 2015 ) n=2015)

And For

lim n 2 sin 2 π n \displaystyle \lim_{ n \rightarrow \infty} \dfrac{2}{\sin\frac{2\pi}{n}}

= n π \boxed{\dfrac{n}{\pi}}

Vraj Mehta - 6 years, 3 months ago

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@Vraj Mehta So you agree with me that the answer is not accurate?

Raghav Vaidyanathan - 6 years, 3 months ago

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@Raghav Vaidyanathan The Answer I Gave Is Exact As I used Proper Method Which is Quite Long.

But The Error In your Answer And My Answer Is 1.6 1 0 6 1.6*10^{-6}

So it can be Ignored

Vraj Mehta - 6 years, 3 months ago

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