Solving a Math Competition Problem: Part 7

This series of posts concerns solving the following problem from the 2016 University of Maryland High School Mathematics Competition.

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. 2^8

b. 2^9

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.)

OK, so I wanted to prove that each region would be a triangle. So I decided to project the sphere onto a plane.

The projection of four planes:

After a while, I had a chart for max possible regions.

  • 1 plane: Max regions = 2
  • 2 planes: Max regions = 4
  • 3 planes: Max regions = 8 (exponential?)
  • 4 planes: Max regions = 14 (nope!)
  • 5 planes: Max regions = 22 (huh?)

Then, really because I had no other ideas, I tried counting intersection points AND max regions
(remembering that one intersection point is “at infinity” – that is, the north pole).

  • 1 plane: Intersection Points = 0, Max regions = 2
  • 2 planes: Intersection Points = 2, Max regions = 4
  • 3 planes: Intersection Points = 6, Max regions = 8
  • 4 planes: Intersection Points = 12, Max regions = 14
  • 5 planes: Intersection Points  20, Max regions = 22

Oh. My. Goodness. The max regions are simply the number of intersection points plus 2. Could it really REALLY be that simple?

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