finding r, absolute value of (sqrt3 + i)

**BugzLooney** · January 1st 2011, 02:09 PM

I was doing some reading through the forum and noticed a question about De Moivre's theorem and realized that I don't know how to find r at all.

I was wondering why the radius is 2 when you take the absolute value of (sqrt3 + i)

**pickslides** · January 1st 2011, 02:12 PM

For $\displaystyle x+yi \implies r =\sqrt{x^2+y^2}$

so $\displaystyle \sqrt{3}+i \implies r =\sqrt{(\sqrt{3})^2+1^2} =\sqrt{4}=2$

**BugzLooney** · January 1st 2011, 02:27 PM

Is this the form that all of these types of questions are in? I mean would it be possible to see a question asking for the absolute value of (sqrt3 + i) without any hints of r or anything?

**pickslides** · January 1st 2011, 02:29 PM

Any question where you need to find the solutions of a complex equation using De Moivre's would assume $r$ is needed.

**Archie Meade** · January 1st 2011, 02:55 PM

Originally Posted by BugzLooney

Is this the form that all of these types of questions are in? I mean would it be possible to see a question asking for the absolute value of (sqrt3 + i) without any hints of r or anything?

You could be required to calculate the "modulus" of a complex number.

On an Argand Diagram, a complex number's "modulus" is it's distance from the origin $(0,0)$ .

Pythagoras' theorem calculates this
[you can draw a right-angled triangle with hypotenuse being the line from $(0,0)$ to $z$ ].

$z=a+ib$

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

**rtblue** · January 1st 2011, 03:15 PM

For any further questions you might have, I find this video to be very informative:

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