Express the complex number in trigonometric form.

-5i

Question

Express the complex number in trigonometric form. 

-5i

anonymous · Answer

5(cos 90° + i sin 90°)

anonymous · Answer

my answer

praxer · Answer

$$\huge 5(\cos270+i\sin270)$$.

I think this should be the answer. Not sure..

anonymous · Answer

I should be 5(cos90+isin(-90))
The angle can only take values from [-180,180) degrees.

anonymous · Answer

which one is right

anonymous · Answer

im i right?

anonymous · Answer

You're close, I have shown my answer to you. :D

praxer · Answer

@kainui, can help us confirm. :)

kainui · Answer

@praxer is correct. Usually you only pick numbers between 0 and 2pi, and 5(cos90+isin(-90)) is wrong because you have to have the same angle for both.

Notice that if you had just put -90 degrees in cosine it would have been ok and just as good as 270 degrees since -90+360=270 because it's periodic.

anonymous · Answer

Sorry, it's a typo... The angle should be -90 for both, like you said...
I was referring to the command Arg, which takes values from (-pi,pi] :D

anonymous · Answer

http://mathworld.wolfram.com/ComplexArgument.html For anyone who is interested. :D