Answer
The trigonometric (polar) form of $-4i$ is $4cis270^\circ$ or $4(cos270^\circ + i\cdot sin270^\circ)$.
Work Step by Step
$-4i$ is at $270^\circ$ with absolute value $\sqrt{0^2 + (-4)^2} = 4$, the trigonometric (polar) form of $-4i$ is $4cis270^\circ$ or $4(cos270^\circ + i\cdot sin270^\circ)$.
