Define y = x^4
y^2  y  240 = 0
y = (1 +/ sqrt(1+960)) / 2 = 15 or 16
x^4 = 16 has roots of 2, 2, 2i and 2i, only one of which has a positive real part.
x^4 =  15 has roots 15^(1/4)/sqrt(2) times choice of:
1+i, 1+i, 1i and 1i
Two of these have positive real parts.
All together the equation has 3 roots with positive real parts, 3 with negative real parts and 2 with real part of zero.

Posted by Charlie
on 20161016 11:13:54 