Infinite Cones, Finite Surface Area?

Geometry Level 3

A unit circle is divided into halves. One of these pieces is divided into two halves, and one of those pieces is divided into two halves, and so on. Each of these sectors of the unit circle is rolled up into a cone-shape, and a circular base is capped onto each of the cones. The total surface area S S of all of these cones combined can be expressed as S = a b π S=\dfrac{a}{b}\pi for positive coprime integers a , b a,b . What is a + b a+b ?


The answer is 7.

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1 solution

Daniel Liu
Apr 4, 2014

First, note that the sum of all the lateral faces of the cones is the same as the area of the unit circle, which is π \pi .

Now all we need to do is find the sum of the areas of the circular bases.

We can see that the circumference of the circular bases are 1 2 \dfrac{1}{2} th, 1 4 \dfrac{1}{4} th, 1 8 \dfrac{1}{8} th etc. of the circumference of the unit circle.

Since the circumference of the unit circle is 2 π 2\pi , the circumferences of the bases are π , π 2 . π 4 , \pi,\dfrac{\pi}{2}.\dfrac{\pi}{4},\ldots .

If the circumference of the base is n π n\pi , then the radius is n π 2 π = n 2 \dfrac{n\pi}{2\pi}=\dfrac{n}{2} , and the area is n 2 π 4 \dfrac{n^2\pi}{4} .

Therefore, the sum of the areas of all the bases is π 4 + π 16 + π 64 + = π 4 1 1 4 = π 3 \dfrac{\pi}{4}+\dfrac{\pi}{16}+\dfrac{\pi}{64}+\cdots = \dfrac{\dfrac{\pi}{4}}{1-\dfrac{1}{4}}=\dfrac{\pi}{3} .

Our final answer, is therefore π + π 3 = 4 3 π \pi+\dfrac{\pi}{3}=\dfrac{4}{3}\pi , and our answer is 4 + 3 = 7 4+3=\boxed{7} .

I used the same method. I liked the problem, even if it wasn't hard. Keep up the work Daniel!

Justin Wong - 7 years, 2 months ago

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Thanks! It was just a nice idea that I came up with. As a challenge, see if you can find the sum of the volumes.

Daniel Liu - 7 years, 2 months ago

Hey dude! I LOVED this problem, unfortunately didn't get it out of simple computation error. Anyways, I've noticed a SIGNIFICANT change in the quality of the problems and solutions that you've been posting lately! Lot's of REALLY fantastic problems as well as astounding solutions! I think right now you're the best user on the site! Calvin never does anything, Daniel Chiu is never on, Dave Mattingly hardly ever posts, etc.. Keep it up! :D

Finn Hulse - 7 years, 2 months ago

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Thanks! I really appreciate your support! However, you claiming other people don't do good now doesn't make me feel very happy. For example, Calvin certainly does stuff. He recently created multiple level 5 questions; he just decided that posting them in a set all at once is better than posting them publicly one by one.

But really, I feel really happy from your feedback! Thank you!

Daniel Liu - 7 years, 2 months ago

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:D I just meant that he isn't as active as he has been in the past.

Finn Hulse - 7 years, 2 months ago

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