Let P(x)=z^4-2z^3+7z^2-4z+10
P(ai)=a^4+2a^3.i+7a^2-4ai+10
P(-ai)=a^4-2a^3+7a^2+4ai+10
P(ai)+P(-ai)=2(a^4-7a^2+10)=0
(a^2-5)(a^2-2)=0
a=+-sqrt(5) and +-sqrt(2)
Let A,B,C,D be roots of P(z):
A+B+C+D=2
C+D=2 and CD=5 or 2
C^2+D^2=(C+D)^2-2CD
ie. C^2+D^2=-6 or 0 but we know C=/=+-iD as...