Line 3: Line 3:
  
 
<center> <math> \frac{a+b}{a} = \frac{a}{b} </math> </center>
 
<center> <math> \frac{a+b}{a} = \frac{a}{b} </math> </center>
 
We can solve this equation to find an explicit quantity for the ratio.
 
 
<center> <math> LHS = \frac{a}{b} + \frac{b}{a} = 1 + \frac{b}{a} </math> </center>
 
 
<center> <math> 1 + \frac{b}{a} = \frac{a}{b} </math> </center>
 
 
We set the ratio equal to a certain quantity given by r.
 
 
<center> <math> r = \frac{a}{b} </math> </center>
 
 
Then we can solve for the ratio numerically.
 
 
<center> <math> 1 + \frac{1}{r} = r </math> </center>
 
 
<center> <math> r + 1 = r^2 </math> </center>
 
 
We can see from the above result that the golden ratio can also be described as a ratio such that in order to get the square of the ratio, you add one to the ratio.
 
 
<center> <math> r^2 - r - 1 = 0 </math> </center>
 
 
We can then apply the quadratic formula to solve for the roots of the equation.
 
 
<center> <math> r = \frac{1 \pm \sqrt{1^2 - 4(1)(-1)}}{2} = \frac{1 \pm \sqrt{5}}{2} </math> </center>
 
 
The positive root is then the golden ratio.
 
  
 
<center> <math> \frac{1 \pm \sqrt{5}}{2} = \phi </math> </center>
 
<center> <math> \frac{1 \pm \sqrt{5}}{2} = \phi </math> </center>

Revision as of 16:43, 2 December 2018

Introduction

The golden ratio is a ratio such that, given two quantities a and b,

$ \frac{a+b}{a} = \frac{a}{b} $
$ \frac{1 \pm \sqrt{5}}{2} = \phi $

The golden ratio, $ \phi $, is sometimes also called the golden mean or the golden section. The golden ratio can be frequently observed in man-made objects, though they are generally “imperfectly golden” – that is, the ratio is approximately the golden ratio, but not exactly. Some everyday examples include: credit cards, $ \frac{w}{h}=1.604 $, and laptop screens, $ \frac{w}{h}=1.602 $ (Tannenbaum 392).

Visualizations of the golden ratio can be seen below (Weisstein):

Visualization 1.png
Visualization 2.png

MathIsFun also has an interactive display that can construct a rectangle in the golden ratio given a certain fixed width or length.

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Basic linear algebra uncovers and clarifies very important geometry and algebra.

Dr. Paul Garrett