Creating a Triangle based on Phi (or Pythagoras meets Fibonacci):
Pythagoras discovered that a right triangle with sides of length a and b and a hypotenuse of length c has the following relationship:
a² + b² = c²
A foundational equality of phi has a similar structure:
1 + Phi = Phi2
( 1+ 1.618… = 2.618… )
By taking the square root of each term in this equality, we have the dimensions of a triangle, known as a Kepler Triangle, a right triangle based on this phi equality, where:
|Side||Length squared |
or square root
|Length divided |
by phi so c = 1
|a||1||1||1 / Phi|
|b||Phi||√ Phi||1 / √ Phi|
This triangle is illustrated below. It has an angle of 51.83° (or 51°50′), which has a cosine of 0.618 or phi.
The Pythagorean 3-4-5 triangle is the only right-angle triangle whose sides are in an arithmetic progression. 3 + 1 = 4, and 4 plus 1 = 5. The Kepler triangle is the only right-angle triangle whose side are in a geometric progression: The square root of phi times Φ = 1 and 1 times Φ = Φ.
Although difficult to prove with certainty due to deterioration through the ages, this angle is believed by some to have been used by the ancient Egyptians in the construction of the Great Pyramid of Cheops.
Other triangles with Golden Ratio proportions can be created with a Phi (1.618 0339 …) to 1 relationship of the base and sides of triangles:
No three successive numbers in the Fibonacci series can be used to create a right triangle. Marty Stange, however, contributed the following relationship in January 2007: Every successive series of four Fibonacci numbers can be used to create a right triangle, with the base and hypotenuse being determined by the second and third numbers, and the other side being the square root of the product of the first and fourth numbers. The table below shows how this relationship works:
Thus for the illustration highlighted in gold, Stange’s Treatise on Fibonacci Triangles reveals that a triangle with sides of 5 and the square root of 39 (e.g., 3 x 13) will produce a right triangle with a hypotenuse of 8.
As greater numbers in the series are used, the triangle approaches the proportions of the phi-based Kepler Triangle above, with a ratio of the hypotenuse to the base of Phi, or 1.618…