# Math Help - HELPP Write complex number in polar form.

1. ## HELPP Write complex number in polar form.

How would I write -4i in polar form? I know that i solve for y..and I get for but I'm not sure where to go from there.

2. For any complex number $a + bi$, it can be expressed in the form $\mbox{r cis} \,\,\theta$.

$r = \sqrt{a^2 + b^2}$

$\theta = \arctan{\left(\frac{b}{a}\right)}$

For your question however you should be able to see the number lies on the im axis and so the angle is $\frac{-\pi}{2}$ and there is no real part so $r = 4$

3. $- 4i = 4\left( {\cos \left( {\frac{{ - \pi }}{2}} \right) + i\sin \left( {\frac{{ - \pi }}{2}} \right)} \right) = 4e^{i\frac{{ - \pi }}{2}}$