>>55
x^3 -3x +1 = (x-α) (x-(α-1)/α) (x+1/(α-1)),

α　　　= 2cos(40°) =　1.532088886
(α-1)/α = 2cos(-80°) =　0.347296355
-1/(α-1) = 2cos(160°) = -1.879385241