4 Folds, 40 Theorems, and Chinese New Year

February 5, 2016 at 11:50 am Leave a comment

If you asked a mathematician how to make an origami coin purse for Chinese New Year, you might receive the following instructions:

  1. Rotate a square piece of paper 45°.
  2. Reflect an isosceles right triangle about a diagonal of the square so that congruent isosceles right triangles coincide.
  3. Reflect the lower left vertex of the isosceles right triangle so it coincides with the opposite leg, such that the base of the isosceles right triangle and one side of the newly formed isosceles acute triangle are parallel.
  4. Reflect the other vertex of the isosceles right triangle to coincide with a vertex of the isosceles triangle formed in the previous step.
  5. Fold down a smaller isosceles right triangle from the top.
  6. Fill the coin purse with various denominations of currency, such as a $2 bill, a $1 gold coin, a quarter, or a penny.

Don’t like those directions? Perhaps these pictures will help.

Coin Purse Instructions
To be truly authentic for Chinese New Year, you’ll want to use red paper instead of white.

The Common Core State Standards for Mathematics (CCSSM) have added rigor to geometry by requiring students to prove no fewer than a dozen different theorems. Further, the geometry progressions document formulated by Dr. Hung-Hsi Wu suggests that a high school geometry course should use a transformational approach, basing a majority of the foundational axioms on rotations, translations, and reflections.

What struck me about the origami coin purse was the myriad geometry concepts that could be investigated during this simple paper-folding activity. Certainly, the idea to use origami in geometry class in nothing new, but this one seems to have a particularly high return on investment. Creating the coin purse requires only four folds, and though perhaps I lied by claiming forty theorems in the title, the number of axioms, definitions, and theorems that can be explored is fairly high. (Claiming “forty theorems” wasn’t so much untruthful as much as it was archaic. The word forty has been used to represent a large but unknown quantity, especially in religious stories or folk tales, as in Ali Baba and the 40 Thieves, it rained “40 days and 40 nights,” or Moses and the Israelites wandered the desert for 40 years.)

For instance, the fold in Step 3 requires parallel lines, and the resulting figures can be used to demonstrate and prove that parallel lines cut by a transversal result in alternate interior angles that are congruent.

Alternate Interior Angles

No fewer than four similar isosceles right triangles are formed, and many congruent isosceles right triangles result.

The folded triangle in the image above is an isosceles acute triangle; prove that it’s isosceles. And if you unfold it from its current location, the triangle’s previous location and the unfolded triangle together form a parallelogram; prove it.

The final configuration contains a pair of congruent right triangles; you can use side-angle-side to prove that they’re congruent.

Isosceles Right Triangles

But maybe you’re not into proving things. That’s all right. The following are all of the geometric terms that can be used when describing some facet of this magnificent creation.

  • Parallel
  • Congruent
  • Similar
  • Alternate
  • Angle
  • Square
  • Reflection
  • Rotation
  • Translation
  • Transversal
  • Bisect
  • Coincide
  • Right Angle
  • Perpendicular
  • Isosceles
  • Vertex
  • Triangle
  • Trapezoid
  • Parallelogram
  • Pentagon

And the following theorems and axioms can be proven or exemplified with this activity.

  • Sum of angles of a triangle is  180°.
  • Alternate interior angles formed by a transversal are congruent.
  • Any quadrilateral with four congruent sides is a parallelogram.
  • Congruent angles are formed by an angle bisector.
  • Triangles with two sides and the included angle congruent are congruent.

There is certainly more to be found. What do you see?


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The Math Jokes 4 Mathy Folks blog is an online extension to the book Math Jokes 4 Mathy Folks. The blog contains jokes submitted by readers, new jokes discovered by the author, details about speaking appearances and workshops, and other random bits of information that might be interesting to the strange folks who like math jokes.

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