How Do the Roots of Complex Numbers Vary in Distribution?

[phospho]

say we had a complex number w^4 such that w^4 = -8 +i8\sqrt{3} so w = 2(cos(\frac{\pi}{6} + \frac{k\pi}{2}) + isin(\frac{\pi}{6} + \frac{k\pi}{2})) where k is an integer

in a question I was asked to find the roots of w, as there will be 4 my first assumption is that the roots would be spread throughout the argand diagram, i.e the first root would be \sqrt{3} + i and the second would be -\sqrt{3} + i and so on till I get the 4 roots. However this is not the case, as if I substitute values for k, I get the roots to be \sqrt{3} + i, -1 + \sqrt{3}i, 1 - \sqrt{3}i, -\sqrt{3} - i which is actually correct.

However in the question below, the roots do seem to be spread evenly:


Prove that cos\frac{\pi}{12} = m and sin\frac{\pi}{12} = n, where m = \frac{\sqrt{3} + 1}{2\sqrt{2}} and n = \frac{\sqrt{3} -1}{2\sqrt{2}}
Hence find in terms of m and n, in the form a + ib, where a,b are real, the fourth roots of 4(cos(\frac{\pi}{3}) + isin(\frac{\pi}{3}))

the first root is z = \sqrt{2}m + i\sqrt{2}n second -\sqrt{2}m + i\sqrt{2}n and so on...

why is it in one question the 4 roots are not spread evenly and I have to adjust k to find the roots, while in the other they are spread evenly. How do I spot whether or not they will be spread evenly?

thanks,

 

[tiny-tim]

hi phospho!

the fourth (or nth) roots of any number w⁴ (or wⁿ)will be w times any fourth (or nth) root of 1

ie w, iw, i²w (= -w), and i³w (= -iw)

(so this is wrong …)

Hence find in terms of m and n, in the form a + ib, where a,b are real, the fourth roots of 4(cos(\frac{\pi}{3}) + isin(\frac{\pi}{3}))

the first root is z = \sqrt{2}m + i\sqrt{2}n second -\sqrt{2}m + i\sqrt{2}n

 

[phospho]

hi phospho!

the fourth (or nth) roots of any number w⁴ (or wⁿ)will be w times any fourth (or nth) root of 1

ie w, iw, i²w (= -w), and i³w (= -iw)

(so this is wrong …)

I have the answers in front of me, and they have the same roots that I gave above.

 

[Millennial]

Why don't you simply halve cosine twice? To be more precise, \cos(\pi/3)=1/2 and we have the half-angle formula \cos^2(x)=\frac{\cos(2x)+1}{2}. Applying it twice should give you \cos(\pi/12), without the need of complex numbers at all.