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Complex roots of unity

  • Thread starter Xeract
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Homework Statement



I need to find the real and imaginary roots of z^4 = -1.

The Attempt at a Solution



The polar coordinates of -1 are at (-1, pi), (-1, 3pi) etc so if I assume the solutions take the form z = exp[i n theta] then

n theta = pi + 2npi

This dosen't seem to give the correct roots though, what am I doing wrong? I don't want the solution, just the method so I can work it through for myself if anyone can help.

Thanks
 

Answers and Replies

  • #2
arildno
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Are you familiar with the fact that:

Multiplying two complex numbers yields a complex number whose angle to the positive real axis is the sum of the factors' angles to the same axis (while the modulus/length of the complex number gained is the product of the facturs' moduli)?

Thus, the fourth roots of a complex number must have one fourth the angle that complex number may have, as measured to the positive real axis.

Remember that the number (-1) can be said to have the angles [itex]\pi,3\pi,5\pi,7\pi[/itex] to the positive x-axis.
Ask yourself why these four angular representations are both necessary and sufficient to find ALL fourth roots of (-1)!
 

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