x1=(-3+5^0.5)/2,x2=-(3+5^0.5)/2
x1^3+8x2+20=[(-3+5^0.5)^3-8(3+5^0.5)+20=[-27+9*5^0.5-3*5+5*5^0.5]/8-4(3+5^0.5)+20
=(22-18*5^0.5)/8=(11-9*5^0.5)/4