Solve $x^{2} + 8 x + 20 = 0$ by completing the square

\begin{aligned} x^{2} + 8 x + 20 & = 0 \\ (x + 4)^{2} - 16 + 20 & = 0 \\ (x + 4)^{2} & = - 4 \\ x + 4 & = \pm \sqrt{- 4} \\ x & = - 4 \pm \sqrt{4 \times - 1} \\ x & = - 4 \pm 2 i \end{aligned}

Solve $x^{2} + 8 x + 20 = 0$ using the quadratic formula

\begin{aligned} x & = \frac{- b \pm \sqrt{b^{2} - 4 a c}}{2 a} \\ = \frac{- 8 \pm \sqrt{8^{2} - 4 (1) (20)}}{2 (1)} \\ = \frac{- 8 \pm \sqrt{- 16}}{2} \\ = \frac{- 8 \pm 4 \sqrt{- 1}}{2} \\ = - 4 \pm 2 i \end{aligned}