From Förberedande kurs i matematik 2

If we determine the number's magnitude \displaystyle r and argument \displaystyle \alpha , we can write its polar form using the formula

\displaystyle r\left( \cos \alpha +i\sin \alpha \right)

Because the number lies on the imaginary axis, it is possible to write its magnitude and argument directly:

The polar form is

\displaystyle 11\left( \cos \frac{3\pi }{2}+i\sin \frac{3\pi }{2} \right).