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5/12/13 — Happy Right Triangle Day!

Happy Right Triangle Day!  Today we celebrate a favorite geometric object:  the 5-12-13 right triangle.

5-12-13 Triangle blue

Of course, the sides of this triangle satisfy the Pythagorean Theorem

5^2 + 12^2 = 13^2

but one reason I like this particular right triangle so much is the role it plays in another favorite triangle.  The 5-12-13 triangle fits together perfectly with the 9-12-15 right triangle

5-12-13 plus 9-12-15 Triangle blue


to make the 13-14-15 triangle!

13-14-15 triangle blue

The 13-14-15 triangle is special in its own right:  it is a Heronian triangle, a triangle with rational side lengths and rational area.  In fact, this triangle has  integer side lengths and integer area, making it especially interesting!

Happy Right Triangle Day!  Be sure to marvel at some perpendicularity today.


  1. Simon Gregg says:

    All right, Mr Honner! Happy Right Triangle Day!

    I liked this lots. Looking around I found another way of splitting up the 13-14-15 triangle which I like too.

    Blogged here:

  2. Gerben says:

    You wrote 13^3.

  3. Nat says:

    Happy Right Triangle Day! I think you should also check out the work of Michelle Desmarais. She has done incredible work pertaining to primitive Pythagorean Triples and possible extensions to the Pythagorean theorem.

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