A sphere is divided into regions by 9 planes that are passing through its center. What is the largest possible number of regions that are created on its surface?

a.

b.

c. 81

d. 76

e. 74

This series was actually written by my friend Jeff Cagle, department head for mathematics at Chapelgate Christian Academy, as he tried technique after technique to solve this problem. I thought that his resolution to the problem was an excellent example of the process of mathematical problem-solving, and (with his permission) I am posting the process of his solution here. (For the record, I have no doubt that I would not have been able to solve this problem.)

On my first pass, all I could do was to visualize the first three planes, one at the equator, one passing through the prime meridian in Greenwich England, and one passing through the International Date Line. That gave me regions, so my preliminary conjecture was “b. ”. But I couldn’t prove it. And when I tried to mentally add a fourth plane to my diagram – one starting in Ukraine or something and hitting the equator halfway between the others – I found that I couldn’t clearly see that plane and count the regions formed. That vexed me for a while, and I put it away for the day.

The next day, I realized that I wasn’t going to be able to picture these planes, and I needed to find a way to describe their directions mathematically. The picture I had was of the equatorial plane and a second plane passing through it in the center. That second plane could be rotated any amount around the equator – described by one angle – and then elevated by tilting to a different angle. So I conjectured that two angles uniquely describe each plane: 𝜃 to describe angle around and 𝜙 to describe angle of elevation.

In the shower, I realized that I had just rediscovered latitude and longitude! That made me feel much better about my mathematical description as likely correct.

But now, how to turn the mathematical description into a solution? If I have one plane at (𝜃1,𝜙1), how do I count the regions it creates with the other planes?

I'm a Professor of Mathematics and a University Distinguished Teaching Professor at the University of North Texas. For eight years, I was co-director of Teach North Texas, UNT's program for preparing secondary teachers of mathematics and science.
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