How do I write the trig form of -2i?
mr fantastic means the same thing. $\displaystyle r cis \theta$ is a shorthand way of writing $\displaystyle r(cos \theta + i sin \theta)$, so
$\displaystyle -2i = 2cis \left( -\frac{\pi}{2} \right) = 2\left[cos \left( -\frac{\pi}{2} \right) + i sin \left( -\frac{\pi}{2} \right)\right]$.
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