8. Let $x, y$ be complex numbers such that $\frac{x^{2}+y^{2}}{x+y}=4$ and $\frac{x^{4}+y^{4}}{x^{3}+y^{3}}=2$. Find all possible values of $\frac{x^{6}+y^{6}}{x^{5}+y^{5}}$.
Answer: $10 \pm 2 \sqrt{17}$ Let $A=\frac{1}{x}+\frac{1}{y}$ and let $B=\frac{x}{y}+\frac{y}{x}$. Then
$$
\frac{B}{A}=\frac{x^{2}+y^{2}}{x+y}=4
$$
so $B=4 A$. Next, note that
$$
B^{2}-2=\frac{x^{4}+y^{4}}{x^{2} y^{2}} \text { and } A B-A=\frac{x^{3}+y^{3}}{x^{2} y^{2}},
$$
so
$$
\frac{B^{2}-2}{A B-A}=2
$$

Substituting $B=4 A$ and simplifying, we find that $4 A^{2}+A-1=0$, so $A=\frac{-1 \pm \sqrt{17}}{8}$. Finally, note that
$$
64 A^{3}-12 A=B^{3}-3 B=\frac{x^{6}+y^{6}}{x^{3} y^{3}} \text { and } 16 A^{3}-4 A^{2}-A=A\left(B^{2}-2\right)-(A B-A)=\frac{x^{5}+y^{5}}{x^{3} y^{3}},
$$
so
$$
\frac{x^{6}+y^{6}}{x^{5}+y^{5}}=\frac{64 A^{2}-12}{16 A^{2}-4 A-1}=\frac{4-16 A}{3-8 A}
$$
where the last inequality follows from the fact that $4 A^{2}=1-A$. If $A=\frac{-1+\sqrt{17}}{8}$, then this value equals $10+2 \sqrt{17}$. Similarly, if $A=\frac{-1-\sqrt{17}}{8}$, then this value equals $10-2 \sqrt{17}$.
(It is not hard to see that these values are achievable by noting that with the values of $A$ and $B$ we can solve for $x+y$ and $x y$, and thus for $x$ and $y$.)