Solution. The four roots of $z^{4}+16$ are given by

$$

\begin{aligned}

\sqrt[4]{-16} &=\sqrt[4]{16 e^{\pi i}}=\sqrt[4]{16} e^{\pi i / 4} e^{2 m \pi i / 4} \\

&=2 e^{\pi i / 4}, 2 e^{3 \pi i / 4}, 2 e^{5 \pi i / 4}, 2 e^{7 \pi i / 4}

\end{aligned}

$$

for $m=0,1,2,3$. We see that these roots appear in conjugate pairs:

$$

2 e^{\pi i / 4}=\overline{2 e^{7 \pi i / 4}} \text { and } 2 e^{3 \pi i / 4}=\overline{2 e^{5 \pi i / 4}}

$$

This gives the way to factor $z^{4}+16$ into two quadratic polynomials of real coefficients:

$$

\begin{aligned}

z^{4}+16 &=\left(z-2 e^{\pi i / 4}\right)\left(z-2 e^{3 \pi i / 4}\right)\left(z-2 e^{5 \pi i / 4}\right)\left(z-2 e^{7 \pi i / 4}\right) \\

&=\left(\left(z-2 e^{\pi i / 4}\right)\left(z-2 e^{7 \pi i / 4}\right)\right)\left(\left(z-2 e^{3 \pi i / 4}\right)\left(z-2 e^{5 \pi i / 4}\right)\right) \\

&=\left(z^{2}-2 \operatorname{Re}\left(2 e^{\pi i / 4}\right) z+4\right)\left(z^{2}-2 \operatorname{Re}\left(2 e^{3 \pi i / 4}\right) z+4\right) \\

&=\left(z^{2}-2 \sqrt{2} z+4\right)\left(z^{2}+2 \sqrt{2} z+4\right)

\end{aligned}

$$