# The Golden Ratio

The Golden Ratio
Fields: Algebra and Geometry
Image Created By: Azhao1
Website: The Math Forum

The Golden Ratio

The pentagram at the right is designed using two isosceles triangles that exhibit the golden ratio. One triangle has a base of length 1 and legs of length $\varphi$, while the other triangle has a base of length $\varphi$ and legs of length 1.

The golden number, often referred to as phi is numerically equal to $\frac{1 + \sqrt{5}}{2} \approx 1.61803399 \dots =\varphi$.

The term golden ratio refers to the ratio $\varphi$ : 1.

This page explores real world applications for the golden ratio, common misconceptions about the golden ratio, and multiple derivations of the golden number.

# Basic Description

The golden ratio, approximately 1.618, is called golden because many geometric figures involving this ratio are often said to possess special beauty. Be that true or not, the ratio has many beautiful and surprising mathematical properties. The Greeks were aware of the golden ratio, but did not consider it particularly significant with respect to aesthetics. It was not called the "divine" proportion until the 15th century, and was not called "golden" ratio until the 18th century. Since then, it has been claimed that the golden ratio is the most aesthetically pleasing ratio, and claimed that this ratio has appeared in architecture and art throughout history. Among the most common such claims are that the Parthenon and Leonardo Da Vinci's Mona Lisa uses the golden ratio. Even more esoteric claims propose that the golden ratio can be found in the human facial structure, the behavior of the stock market, and the Great Pyramids. However, such claims have been criticized in scholarly journals (see references at the end of the page) as wishful thinking or sloppy mathematical analysis. Additionally, there is no solid evidence that supports the claim that the golden rectangle is the most aesthetically pleasing rectangle.

### Misconceptions about the Golden Ratio

In his paper, Misconceptions about the Golden Ratio, George Markowsky investigates many claims about the golden ratio appearing in man-made objects and in nature. Specifically, he claims that the golden ratio does not appear in the Parthenon or the Great Pyramids, two of the more common beliefs. He also disputes the belief that the human body exhibits the golden ratio. To read more, click here!

## A Geometric Representation

### The Golden Ratio in a Line Segment

The golden ratio can be defined using a line segment divided into two sections, of lengths a and b, respectively. If a and b are appropriately chosen, the ratio of a to b is the same as the ratio of a + b to a and both ratios are equal to $\varphi$. The value of this ratio turns out not to depend on the particular values of a and b, as long as they satisfy the proportion. The line segment above exhibits the golden proportions. The golden rectangle is made up of line segments exhibiting the golden proportion. Remarkably, when a square is cut off of the golden rectangle, the remaining rectangle also exhibits the golden proportions. This continuing pattern is visible in the golden rectangle above.

### Triangles

The golden ratio $\varphi$ is used to construct the golden triangle, an isoceles triangle that has legs of length $\varphi$ and base length of 1. It is above and to the left. Similarly, the golden gnomon has base ${\varphi}$ and legs of length 1. It is shown above and to the right. These triangles can be used to form pentagrams and pentacles.

The pentacle below, generated by the golden triangle and the golden gnomon, has many side lengths proportioned in the golden ratio.

$\frac{\mathrm{blue} }{\mathrm{red} } = \frac{\mathrm{red} }{\mathrm{green} } = \frac{\mathrm{green} }{\mathrm{pink} } = \varphi .$

These triangles can be used to form fractals and are one of the only ways to tile a plane using pentagonal symmetry. Two fractal examples are shown below.

# A More Mathematical Explanation

Note: understanding of this explanation requires: *Algebra, Geometry

# Mathematical Representations of the Golden Ratio

## An Algebraic Representation

We may algebraically solve for the ratio ($\varphi$) by observing that ratio satisfies the following property by definition:

$\frac{b}{a} = \frac{a+b}{b} = \varphi$

Let $r$ denote the ratio :

$r=\frac{a}{b}=\frac{a+b}{a}$.

So

$r=\frac{a+b}{a}=1+\frac{b}{a} =1+\cfrac{1}{a/b}=1+\frac{1}{r}$.
$r=1+\frac{1}{r}$

Multiplying both sides by $r$, we get

${r}^2=r+1$

which can be written as:

$r^2 - r - 1 = 0$.

Applying the quadratic formula , we get $r = \frac{1 \pm \sqrt{5}} {2}$.

Because the ratio has to be a positive value,

$r=\frac{1 + \sqrt{5}}{2} \approx 1.61803399 \dots =\varphi$.

## Continued Fraction Representation and Fibonacci Sequences

The golden ratio can also be written as what is called a continued fraction by using recursion.

We have already solved for $\varphi$ using the following equation:

${\varphi}^2-{\varphi}-1=0$.

We can add one to both sides of the equation to get

${\varphi}^2-{\varphi}=1$.

Factoring this gives

$\varphi(\varphi-1)=1$.

Dividing by $\varphi$ gives us

$\varphi -1= \cfrac{1}{\varphi }$.

Solving for $\varphi$ gives

$\varphi =1+ \cfrac{1}{\varphi }$.

Now use recursion and substitute in the entire right side of the equation for $\varphi$ in the bottom of the fraction.

$\varphi = 1 + \cfrac{1}{1 + \cfrac{1}{\varphi } }$

Substituting in again,

$\varphi = 1 + \cfrac{1}{1 + \cfrac{1}{1 + \cfrac{1}{\varphi}}}$

$\varphi = 1 + \cfrac{1}{1 + \cfrac{1}{1 + \cfrac{1}{\cdots}}}$

This last infinite form is a continued fraction

If we evaluate truncations of the continued fraction by evaluating only part of the continued fraction (the finite displays above it), replacing $\varphi$ by 1, we produce the ratios between consecutive terms in the Fibonacci sequence.

$\varphi \approx 1 + \cfrac{1}{1} = 2$

$\varphi \approx 1 + \cfrac{1}{1+\cfrac{1}{1}} = 3/2$

$\varphi \approx 1 + \cfrac{1}{1 + \cfrac{1}{1 + \cfrac{1}{1} } } = 5/3$

$\varphi \approx 1 + \cfrac{1}{1 + \cfrac{1}{1 + \cfrac{1}{1+\cfrac{1}{1}}}} = 1 + \cfrac{1}{1 + \cfrac{1}{1 + \cfrac{1}{2}}} =1 + \cfrac{1}{1 + \cfrac{2}{3}} = 8/5$

Thus we discover that the golden ratio is approximated in the Fibonacci sequence.

$1,1,2,3,5,8,13,21,34,55,89,144...\,$

 $1/1$ $=$ $1$ $2/1$ $=$ $2$ $3/2$ $=$ $1.5$ $8/5$ $=$ $1.6$ $13/8$ $=$ $1.625$ $21/13$ $=$ $1.61538462...$ $34/21$ $=$ $1.61904762...$ $55/34$ $=$ $1.61764706...$ $89/55$ $=$ $1.61818182...$

$\varphi = 1.61803399...\,$

As you go farther along in the Fibonacci sequence, the ratio between the consecutive terms approaches the golden ratio. Many real world applications of the golden ratio are related to the Fibonacci sequence. For more real-world applications of the golden ratio click here!

In fact, we can prove this relationship using mathematical Induction.

Since we have already shown that

$\varphi = 1 + \cfrac{1}{1 + \cfrac{1}{1 + \cfrac{1}{\cdots}}}$,

we only need to show that each of the terms in the continued fraction is the ratio of Fibonacci numbers as shown above.

First, let $x_1=1$, $x_2=1+\frac{1}{1}=1+\frac{1}{x_1}$, $x_3= 1+\frac{1}{1+\frac{1}{1}}=1+\frac{1}{x_2}$ and so on so that $x_n=1+\frac{1}{x_{n-1}}$.

These are just the same truncated terms as listed above. Let's also denote the terms of the Fibonacci sequence as $f_n=f_{n-1}+f_{n-2}$ where $f_1=1$,$f_2=1$, and so $f_3=1+1=2$, $f_4=1+2=3$ and so on.

We want to show that $x_n=\frac{f_{n+1}}{f_n}$ for all n.

First, we establish our base case. We see that $x_1=1=\frac{1}{1}=\frac{f_2}{f_1}$, and so the relationship holds for the base case.

Now we assume that $x_k=\frac{f_{k+1}}{f_{k}}$ for some $1 \leq k < n$ (This step is the inductive hypothesis). We will show that this implies that $x_{k+1}=\frac{f_{(k+1)+1}}{f_{k+1}}=\frac{f_{k+2}}{f_{k+1}}$.

By our definition of $x_n$, we have

$x_{k+1}=1+\frac{1}{x_k}$.

By our inductive hypothesis, this is equivalent to

$x_{k+1}=1+\frac{1}{\frac{f_{k+1}}{f_{k}}}$.

Now we only need to complete some simple algebra to see

$x_{k+1}=1+\frac{f_k}{f_{k+1}}$

$x_{k+1}=\frac{f_{k+1}+f_k}{f_{k+1}}$

Noting the definition of $f_n=f_{n-1}+f_{n-2}$, we see that we have

$x_{k+1}=\frac{f_{k+2}}{f_{k+1}}$

Since that was what we wanted to show, we see that the terms in our continued fraction are represented by ratios of Fibonacci numbers.

The exact continued fraction is $x_{\infty} = \lim_{n\rightarrow \infty}\frac{f_{n+1}}{f_n} =\varphi$.

## Proof of the Golden Ratio's Irrationality

Remarkably, the Golden Ratio is irrational, despite the fact that we just proved that is approximated by a ratio of Fibonacci numbers. We will use the method of contradiction to prove that the golden ratio is irrational.

Suppose $\varphi$ is rational. Then it can be written as fraction in lowest terms $\varphi = b/a$, where a and b are integers.

Our goal is to find a different fraction that is equal to $\varphi$ and is in lower terms. This will be our contradiction that will show that $\varphi$ is irrational.

First note that the definition of $\varphi = \frac{b}{a}=\frac{a+b}{b}$ implies that $b > a$ since clearly $b+a>b$ and the two fractions must be equal.

Now, since we know

$\frac{b}{a}=\frac{a+b}{b}$

we see that $b^2=a(a+b)$ by cross multiplication. Writing this all the way out gives us $b^2=a^2+ab$.

Rearranging this gives us $b^2-ab=a^2$, which is the same as $b(b-a)=a^2$.

Dividing both sides of the equation by $(b-a)$ and $a$ gives us that

$\frac{b}{a}=\frac{a}{b-a}$.

Since $\varphi=\frac{b}{a}$, we can see that $\varphi=\frac{a}{b-a}$.

Since we have assumed that a and b are integers, we know that b-a must also be an integer. Furthermore, since $a, we know that $\frac{a}{b-a}$ must be in lower terms than $\frac{b}{a}$.

Since we have found a fraction of integers that is equal to $\varphi$, but is in lower terms than $\frac{b}{a}$, we have a contradiction: $\frac{b}{a}$ cannot be a fraction of integers in lowest terms. Therefore $\varphi$ cannot be expressed as a fraction of integers and is irrational.

• Markowsky. “Misconceptions about the Golden Ratio.” College Mathematics Journal. Vol 23, No 1 (1992). pp 2-19.

# References

1. "Parthenon", Retrieved on 16 May 2012.

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