
Mark M. answered 02/04/16
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For a Golden Rectangle:
length : width :: 1.618 : 1
length : 11 : 1.618 : 1 product of means/extremes
length = 11(1.618)
length = 17.798
This is an approximation (but then if you are measuring what else would you need?).
For an exact value (using √5), substitute (1+√5)/2 for 1.618.