I know how to plot an Argand diagram and I think I know how to find the argument usually, but I've done some looking around to be certain of my answers and found what seems to be conflicting information regarding the argument of purely imaginary numbers (for example, $-2i$).
My textbook gives the example of $\arg(-5j) = -\pi/2$, which makes sense to me as I was told to take the angle as a negative, measured clockwise and in radians. On the diagram, a pure negative imaginary number would be $-90$ degrees ($-\pi/2$ radians) from the real numbers line.
However, I've done some looking online and found two sources which insist it's $3*\pi/2$, and I'm not sure why. Links provided: http://www.convertalot.com/complex_arithmetical_calculator.html http://www.mathamazement.com/Lessons/Pre-Calculus/06_Additional-Topics-in-Trigonometry/complex-numbers-in-polar-form.html (Example 1 d).
I'm guessing my textbook is correct as it seems to make the most sense and would be the more trustworthy source, but I have a feeling I'm missing something important considering two different sources came to the same conclusion.