# Q: What is The Golden Ratio? How is it used in Mathematics?

Physicist: The golden ratio, g, is $g=\frac{1+\sqrt{5}}{2}\approx1.62$.
The golden ratio is defined in many (equivalent) ways but the best known is: if A and B are two numbers such that the ratio of A+B to A is equal to the ratio of A to B, then g=A/B.

A rectangle, where the ratio of the long side to the short side is g, is called the “golden rectangle”.  The ancient Greeks, being creepy numerologists with nothing better to do, were really excited about the golden rectangle and worked it into a lot of their art and architecture.

The Golden Rectangle: Clearly, it's the most perfect rectangle ever.

Setting the ratios equal to each other you can solve for g (approx. 1.62): $\begin{array}{ll}\frac{A+B}{A}=\frac{A}{B}\Rightarrow 1+\frac{B}{A}=\frac{A}{B}\Rightarrow 1+\frac{1}{g}=g\Rightarrow g+1=g^2\\\Rightarrow 0=g^2-g-1\end{array}$

It’s this definition, the solution of this equation (0=x2-x-1), that is usually the vehicle that brings g into the conversation (In math circles they usually say “mathversation”).  Or, to put it another way, it shows up by coincidence.

For example; the Fibonacci sequence is a string of numbers, F0, F1, F2, … such that FN = FN-1 + FN-2 and F0 = 0 and F1 = 1.  You can see quickly that the string of numbers is 0, 1, 1, 2, 3, 5, 8, 13, 21, …

It takes a bit of work, but (after some math happens) it turns out that $F_N \approx \frac{1}{\sqrt{5}}g^N$ (round to the nearest integer).  So what does the golden ratio have to do with the Fibonacci sequence?  Not a damn thing, really.  It’s just that about halfway through the derivation you’ll find yourself staring at a “0=x2-x-1″.

“g” showed up a lot for the ancient Greeks because they spent a lot of time playing around with straight edges and compasses.  The equation that describes a circle is quadratic, and the equation that describes a straight line is linear.  So when you’re trying to figure out where an intersection is, or how long a line segment is, you’ll be solving quadratic equations and “0=x2-x-1″ (being simple) is one of the equations you’ll frequently see.

One example of the relationship between straight-edge-and-compass geometry, and the exciting world of quadratic equations.

Finally, g showed up again a couple hundred years ago in the study of “continued fractions“.  However, I personally have only seen continued fractions used exactly once (in the context of rational number approximation) and g was no where to be seen.  The numbers that mathematicians are most excited about these days are: 0, 1, e, and π.

This entry was posted in -- By the Physicist, Equations, Geometry, Math, Philosophical. Bookmark the permalink.