Answer
The equivalent trigonometric form of $-20 + 21i$ is $29cis133.60^\circ$ or $29(cos133.60^\circ + isin133.60^\circ)$.
Work Step by Step
For $-20 + 21i$, the absolute value is $\sqrt{(-20)^2 + 21^2} = 29$ and $\theta = arctan(\frac{21}{-20}) = 133.60^\circ$, since $-20 + 21i$ is in the 2nd quadrant.
Therefore, the equivalent trigonometric form of $-20 + 21i$ is $29cis133.60^\circ$ or $29(cos133.60^\circ + isin133.60^\circ)$.
