Find a and b for complex number

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August 11th 2011, 04:25 AM
fuzzy

Find a and b for complex number

(a+bi)^2=4+4sqrt3i

I got

a^2-b^2+2abi=4+4sqrt3i

a^2=b^2+4
a=sqrt(b^2+4)

2ab=4sqrt3
a=4sqrt3/2b

I can't seem to get the answer when I compare the values

The answer given for a is +/- sqrt 6 and b is +/- sqrt2
August 11th 2011, 04:36 AM
HallsofIvy

Re: Find a and b for complex number

Quote:

Originally Posted by fuzzy

(a+bi)^2=4+4sqrt3i

I got

a^2-b^2+2abi=4+4sqrt3i

a^2=b^2+4
a=sqrt(b^2+4)

2ab=4sqrt3
a=4sqrt3/2b

$a= 2\sqrt{3}/b$
I would avoid square roots and start from this. Now $a^2= 12/b^2$ so that $a^2+ b^2= 4$ becomes $\frac{12}{b^2}+ b^2= 4$ . Multiply both sides by $b^2$ to get $12+ b^4= 4b^2$ of $(b^2)^2- 4b^2+ 12= 0$ .

That is a quadratic equation of $b^2$ - and easily factorable. You will find two values for $b^2$ so 4 different values for b.

Quote:

I can't seem to get the answer when I compare the values

The answer given for a is +/- sqrt 6 and b is +/- sqrt2
August 11th 2011, 04:43 AM
Prove It

Re: Find a and b for complex number

This might be easiest to solve using polars.

$\displaystyle (a + ib)^2 = 4 + 4\sqrt{3}i = 8e^{i\frac{\pi}{3}}$ , so

$\displaystyle \begin{align*}a + ib &= \pm\left(8e^{i\frac{\pi}{3}}\right)^{\frac{1}{2}} \\ &= \pm 2\sqrt{2}e^{i\frac{\pi}{6}}\\ &= \pm 2\sqrt{2}\left(\cos{\frac{\pi}{6}} + i\sin{\frac{\pi}{6}}\right) \\ &= \pm 2\sqrt{2}\left(\frac{\sqrt{3}}{2} + \frac{1}{2}i\right) \\ &= \pm\left(\sqrt{6} + i\sqrt{2}\right) \end{align*}$

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