# Help with complex number derivation

1. Sep 6, 2011

### diracy

1. The problem statement, all variables and given/known data
(a) Suppose the segment connecting (a,b) to (0,0) has length r$_{1}$ and forms an angle $\theta$$_{1}$ with the positive side of the x-axis. Suppose the segment connecting (c,d) to (0,0) has a length r$_{2}$ and forms an angle $\theta$$_{2}$ with the positive side of the x-axis. Now let (a+bi)(c+di)=x+yi. Show that the length of the segment connecting (x,y) to the origin is r$_{1}$r$_{2}$ and the angle formed is $\theta$$_{1}$+$\theta$$_{2}$.

(b) Use the result from (a) to find a complex number z$\in$C such that z^2=i.

2. Relevant equations

3. The attempt at a solution
(a+bi)(c+di)=x+yi
ac+adi+bci+bd(i$^{2}$)=x+yi

I'm not sure where to go from here. Just looking for some help. Thanks!

2. Sep 6, 2011

### eumyang

Remember the formulas for going from standard form of a complex number to trig form. If z = a + bi = r(cos θ + i sin θ), then
$r = \sqrt{a^2 + b^2}$,
a = r cos θ and b = r sin θ, and
$\tan \theta = \frac{b}{a}$.

If x=(ac-bd) and y=(ad+bc), then what is
$\sqrt{x^2 + y^2}$? You'll need to make it equal to r1r2.

For the angle, you'll need the tangent of a sum identity.
$\tan (\theta_1 + \theta_2) = \frac{\tan \theta_1 + \tan \theta_2}{1 - \tan \theta_1 \tan \theta_2}$

Last edited: Sep 6, 2011
3. Sep 7, 2011

### diracy

I'm honestly not getting very far with this. Could you help me out a little more?

4. Sep 7, 2011

### diracy

Ok, I got the first part. Now I need to prove the angle part. Any help?

5. Sep 7, 2011

### eumyang

Start with this. Plug in what x and y equals into the square root and expand the radicand. Show us what you get.

EDIT: Never mind. You said that you got this part.

$\tan \theta_1 = \frac{b}{a}$
and
$\tan \theta_2 = \frac{d}{c}$
Plug these into the formula above and simplify. Somehow you'll have to simplify to y/x. (Remember that you had x and y can be expressed in terms of a, b, c, and d.)

6. Sep 7, 2011

### diracy

Hmmm...

After I plug those in, what methods can I use to simply that expression?

7. Sep 7, 2011

### eumyang

After plugging those in, try multiplying the numerator and denominator of this complex fraction by ac.

(Going to bed now... :zzz:)