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golden spiral : ウィキペディア英語版
golden spiral

In geometry, a golden spiral is a logarithmic spiral whose growth factor is , the golden ratio.〔Chang, Yu-sung, "(Golden Spiral )", The Wolfram Demonstrations Project.〕 That is, a golden spiral gets wider (or further from its origin) by a factor of for every quarter turn it makes.
==Formula==
The polar equation for a golden spiral is the same as for other logarithmic spirals, but with a special value of the growth factor :
:r = ae^\,
or
:\theta = \frac \ln(r/a),
with being the base of Natural Logarithms, being an arbitrary positive real constant, and such that when is a right angle (a quarter turn in either direction):
:e^ \over \theta_\mathrm}.
The numerical value of depends on whether the right angle is measured as 90 degrees or as \textstyle\frac radians; and since the angle can be in either direction, it is easiest to write the formula for the absolute value of b (that is, can also be the negative of this value):
:|b| = \doteq 0.0053468\, for in degrees;
:|b| = \doteq 0.3063489\, for in radians .
An alternate formula for a logarithmic and golden spiral is:
:r = ac^\,
where the constant is given by:
:c = e^b\,
which for the golden spiral gives values of:
:c = \varphi ^ \frac \doteq 1.0053611
if is measured in degrees, and
:c = \varphi ^ \frac \doteq 1.358456.
if is measured in radians.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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