# Math Help - Trig Form

1. ## Trig Form

How do I write the trig form of -2i?

2. Originally Posted by jumpman23
How do I write the trig form of -2i?
You mean the polar form. Have you got class notes or a textbook that give examples?

$-2i = 2 \text{cis} \left( - \frac{\pi}{2}\right)$.

3. No we have not learned about the polar form..

The textbook says to write the trigonometric form of (-2i).

and that it should look something like this: z = r(cosX + isinX)

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

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