Solve \(x^2+8x+20=0\)  by completing the square

\begin{align}x^2+8x+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 2i \end{align}

Solve \(x^2+8x+20=0\)  using the quadratic formula

\begin{align}x & = \frac{-b \pm \sqrt{b^2-4ac}}{2a} \\ & = \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 2i \end{align}