tan36=2m/(1-m^2) where m=tan18
tan72=(4m/(1-m^2)/(1-(4m^2/(1-m^2)^2))
=4m(1-m^2)/(1-m^2-2m)(1-m^2+2m)
4m^2(1-m^2)/(1-m^2-2m)(1-m^2+2m)=1
4m^2(1-m^2)=(1-m^2)^2-4m^2
-4m^4+4m^2=1-2m^2+m^4-4m^2
5m^4-10m^2+1=0

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