The Golden Ratio and Phi
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Picture Credits:

Figures 1&2
http://tsc.k12.in.us/training/CURR/MATH/FIBN/fibnat.htm
  An interesting pattern evolves from the ratios of consecutive Fibonacci numbers. The graph below plots the quotients of the first 13 Fibonacci numbers and their antecedents.
Fig.2
    The ratios of successive Fibonacci numbers approach a value of 1.61803... The limit of the equation as n approaches infinity, is the irrational number, Phi. This value is known as the Golden Ratio. Like the irrational number Pi, the digits of Phi, go on forever without repeating.
The line segment, AB, is divided by a point, C, such that:
Fig. 1
    Since the decimal continues to infinity, the most accurate way to represent Phi is to calculate it algebraically and express it as the root of a quadratic equation. Consider the line segment below:
   Now, we can simplify and substitute to form a quadratic equation that can be solved using the quadratic formula.
  We now see that Phi is actually the positive root of the quadratic equation. The negative root is -phi, such that phi = 0.6180339887499...
    From the quadratic formula, the two roots are:
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References:

Hoggatt, V.E. 1969. Fibonacci and Lucas Numbers. Houghton Mifflin Company, New York. 92 pp.

Runion, Garth E. 1972. The Golden Section and Related Curiosa. Scott, Foresman and Company, Glenview. 150 pp.


http://www.mcs.surrey.ac.uk/Personal/R.Knott/Fibonacci/phi.html

http://pass.maths.org/issue3/fibonacci/

http://www.friesian.com/golden.htm
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