a 36o-72o-72o triangle, with two long sides =
1, and the short side = x .
one of the 72° angles forming two triangles:
isosceles triangle whose short sides are both x , and a smaller 36o-72o-72o
isosceles triangle whose long sides are x , and short side is 1−x
the two 36o-72o-72o triangles are
ignoring the negative answer
phi, the golden ratio<o:p></o:p>
bisect the obtuse angle in the 36o-36o-108o
triangle to get a right
(36o-54o-90o) triangle, with a hypotenuse=(-1+sqrt(5))/2 and the long leg =1/2 .