Hearty Pattern!

Image Credit: VollherzigHerzgeometrie Image Credit: VollherzigHerzgeometrie

Mai's friends and lovers were all invited for the Valentine's Day party. After enjoying the wonderful love music and meal, she believed that it's the great idea to play the heart-counting game. The dialogue started:

Mai : The shape is counted as one heart if:

  • Two adjacent sectors in a heart are either tangential or intersecting.
  • The projected lines are tangent to the sectors.

Yugi : That seems quite interesting.

Tea : I am not even good in discrete mathematics, so I am SO bad in counting. Since I believe you are so good in mathematics, you should figure out the best method to answer Mai's problem! There isn't any reason why I would find another man to love! :)

Mai : The couple who arrives with the correct answer first wins the game and earns a grand prize! The time starts now!

[Mai revealed the image as shown above. After few seconds..]

Yugi : I have the answer, and I am very sure it's correct! [Answer hidden]

Mai : We have a winner!

Tea : Wow! What a versed skill you have here! You are the true Valentine mathematician!

How many hearts in all did he find?

Note: Assume that the common circular sectors have same area, and that the 14-sided polygon is regular.


This is the fifth chapter of the story . Check the following chapter directory if you are interested:

First - Second - Third - Fourth - Fifth


The answer is 70.

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1 solution

Michael Huang
Feb 11, 2017

There are 14 14 large sectors and 14 14 small sectors, where the large ones overlap. There are several cases to consider: the heart is formed by either overlapping sectors or tangential sectors.


Tangential Sectors


For small sectors , the approach is trivial. Since there are clearly 14 14 pairs of adjacent sectors that each form a small heart, we count 14 14 possibilities.

There are two regular heptagons in one regular \(14\)-gon. There are two regular heptagons in one regular 14 14 -gon.

For large sectors , since the large circles overlap two segments of a regular 14 14 -gon, known as tetradecagon , then counting the number of hearts formed by two tangential large circles is equivalent to counting the number of sides of 7 7 -gon or heptagon . In this case, there are 7 7 possibilities. Thus, since there are 2 2 regular heptagons enclosed by a regular 14 14 -gon, there are 7 2 = 14 7 \cdot 2 = 14 possibilities.

So we counted 28 28 hearts of tangential sectors.


Overlapping Sectors


For different-sized sectors , consider the following diagram:

Three sectors indicated by the bolded lines Three sectors indicated by the bolded lines

Because there are 14 14 large sectors adjacent to 2 2 small sectors, we counted 28 28 more hearts.

For two overlapping large sectors , let's now look at the following diagram:

Two large sectors indicated by the bolded lines Two large sectors indicated by the bolded lines

The common region shared by the two sectors indicate the number of hearts, which is congruent to counting the number of midpoints of each segment of 14 14 -gons. Because there are 14 14 midpoints, then there are 14 14 more hearts.

Overall, we counted 42 42 hearts.


Answer


Combining the numbers of hearts from both previous sections, we have 28 + 42 = 70 28 + 42 = \boxed{70} hearts.

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