The Golden Ratio: Phi, 1.618

Phi and Fibonacci in Kepler and Golden Triangles

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
per above
Length,
or square root
Length divided
by phi so c = 1
a 1 1 1 / Phi
b Phi √ Phi 1 / √ Phi
c Phi2 Phi 1

 

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:

The isosceles triangle above on the right with a base of 1 two equal sides of Phi is known as a Golden Triangle.  These familiar triangles are found embodied in pentagrams and Penrose tiles.

 

Creating a Triangle based on Fibonacci numbers

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:

 

Fibonacci Series

The Fibonacci Triangle

b’

a

c

b”

b’xb”

a² + b’xb”

= c²

0

1

1

2

1

0

1

1

1

2

3

1

3

4

1

2

3

5

4

5

9

2

3

5

8

9

16

25

3

5

8

13

25

39

64

5

8

13

21

64

105

169

8

13

21

34

169

272

441

13

21

34

55

441

715

1,156

21

34

55

89

1,156

1,869

3,025

34

55

89

144

3,025

4,896

7,921

55

89

144

233

7,921

12,815

20,736

89

144

233

377

20,736

33,553

54,289

144

233

377

610

54,289

87,840

142,129

 

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…

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