square root of a complex number [Solved!]

X

I cant get a formula for the square root of `a + bi` to work.

Relevant page

<a href="http://stanleyrabinowitz.com/bibliography/complexSquareRoot.pdf">http://stanleyrabinowitz.com/bibliography/complexSquareRoot.pdf</a>

What I've done so far

I start with  `sqrt(a+ b i),` translate into polar or exponential (`re^(i θ)`) coordinates and back again, and get 

`sqrt(a+b i) = sqrt(r) (cos [theta] + i  sin [theta]).`

where 

`theta = 0.5 arcsin (b/r)`

`r =sqrt( (a^2 +b^2)`

Then I test   this with `(2 + 3 i )^2 = -5 + 12 i.`

I get  

`theta = 0.5 arcsin(12/13) = 0.5880`

`sqrt(-5 + 12 i)` ` = sqrt(13)( cos[0.5880 ] + i sin [0.5880]).`

which turns out to be 

` sqrt( 13) ( (.83)+ i (.01) )` 

or about 

`3 + i 0.036` 

which is ridiculous. What is my error ?

Thanks, 

Jedothek

X

@Jedothek: I formatted your question so it was easier to read. I encourage you to make use of the "add math" feature in this forum. (You can click "Show code" to see how I did it.)

(1) I agree with the part where you have:

`cos(0.5880) = 0.83`

However, 

`sin(0.5880) = 0.5547`

So your answer would have been

`sqrt(13)(0.83 + 0.5547i)` ` = 3 + 2i`

(2) Now, the fact our numbers are the wrong way round gives a clue to where the solution went haywire.

The angle representing the complex number `-5+12i` is in the <strong>second quadrant</strong>, so it will be an angle between `pi/2~~1.5708` and `pi~~3.1416`, and `0.5880` is not in this range.

So we need to make use of the <a href="https://www.intmath.com/trigonometric-functions/6-trigonometry-functions-any-angle.php">Reference angle</a> (about half-way down that page).

Our angle should be

`theta = 0.5(pi - arcsin(12/13))` ` = 0.5xx1.9656` ` = 0.9828`

So now we'll have

`sqrt(13)( cos[0.9828] + i sin [0.9828])` ` = 2+3i`

which is what we were hoping for.

Hope it helps.

X

Thanks so much! Here  I  was worrying that mathematics didn't make sense. You have restored  my faith. 
As I'm sure you realized, I must have  been taking that sin .588 in degrees.