Math Is Fun Forum

gairceilatan

Member

Registered: 2007-05-22

Posts: 7

Complex Numbers

Hi there,

I am finding the roots of polynomials using things like, 2+i is a root of the equation hence find all the roots.

Now I am fine with these but I got a question that said the root was purely imaginary, now I took this to mean the root was just i however, this did not work out, and the answer was 2i, so my question is will a purely imaginary root for these types of questions always be 2i? 

Thank you

mathsyperson

Moderator

Registered: 2005-06-22

Posts: 4,900

Re: Complex Numbers

Saying that a number is purely imaginary just means that it doesn't have a real part.

In general, a complex number can be written as x + yi, where x and y are real.
Saying that a complex number in this form is purely imaginary tells you that x = 0.

The imaginary part can still be whatever it likes though. i, 2i, 3i, πi, etc. are all purely imaginary.

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Why did the vector cross the road?
It wanted to be normal.

gairceilatan

Member

Registered: 2007-05-22

Posts: 7

Re: Complex Numbers

Hi mathsyperson,

Thanks for your help, how do you decide what one to use or does it really not matter and when I done it equals to i my calculations were just off?  Or is it specific to the question?

Thank you

lancemaria

Member

Registered: 2007-05-22

Posts: 3

Re: Complex Numbers

hello everyone

mathsyperson

Moderator

Registered: 2005-06-22

Posts: 4,900

Re: Complex Numbers

Hi mathsyperson,

Thanks for your help, how do you decide what one to use or does it really not matter and when I done it equals to i my calculations were just off?  Or is it specific to the question?

Thank you

I'm not sure I get what you're saying here. The answer will depend on what the question is, just like any answer.

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Why did the vector cross the road?
It wanted to be normal.