Golden Gnomon
The golden gnomon is the obtuse isosceles triangle whose ratio of side to base lengths is given by 
, where
is the golden ratio.
Such a triangle has angles of 
-
-
and
can be constructed from a regular pentagon as illustrated
above in red. The corresponding 36-72-72 triangle with side-to-base ratio 
is a golden
triangle.
Golden triangles and gnomons can be dissected into smaller triangles that are golden gnomons and golden triangles (Livio 2003, p. 79).
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golden triangle