Let P and Q be two polynomials given by the list of their coefficients
then rootof(P,Q) gives the value P(α) where α is the
root of Q with largest real part (and largest imaginary part in
case of equality).
In exact computations, Xcas will rewrite rational evaluations of rootof as a unique rootof with degree(P)<degree(Q). If the resulting rootof is the solution of a second degree equation, it will be simplified.
Let α be the root with largest imaginary part of Q(x)=x4+10x2+1 (all roots of Q have real part equal to 0).