SOLUTION: Find the roots of : a) z3 = 1 + i b) z5 = −i c) z7 = −1

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Question 1144148: Find the roots of : a) z3 = 1 + i b) z5 = −i c) z7 = −1
Answer by greenestamps(13200) About Me  (Show Source):
You can put this solution on YOUR website!


Use the "^" character (shift-6) to represent exponentiation (e.g., "z^3", not just "z3"). I assume the roots to be found are for the equations
z%5E3+=+1%2Bi
z%5E5+=+-i
z%5E7+=+-1

Finding roots of complex numbers is made easy with deMoivre's Theorem.

If a complex number is represented in the form

x+=+r%2Acis%28theta%29

where r is a magnitude and theta is an angle of the complex number in the complex plane, then deMoivre's Theorem says that the n n-th roots of the complex number are found as follows:

(1) the magnitude of each root is the n-th root of the magnitude of the given complex number; and
(2) the "primary" n-th root has an angle that is (1/n) times the angle of the given complex number; and the other n-th roots are distributed around the circle at increments of 360/n degrees.

Here is an example....

Find the roots of z%5E4+=+i

The given complex number in the required form is 1%2Acis%2890%29.

The magnitude of the given complex number is 1, so the magnitude of each of the 4-th roots is 1^(1/4)=1.

The angle of the given complex number in the complex plane is 90 degrees, so the "primary" 4-th root of i is at an angle of 90/4 = 22.5 degrees. The other 4-th roots of i are then distributed around the plane at increments of 360/4 = 90 degrees.

So the solutions to the equation z%5E4=i are
(1) 1%2Acis%2822.5%29
(2) 1%2Acis%28112.5%29
(3) 1%2Acis%28202.5%29
(4) 1%2Acis%28292.5%29

Use the theorem to find the solutions to your examples.

To get you started -- in case you need help with this part -- the complex numbers in your examples are
(1) magnitude sqrt(2), angle 45 degrees
(2) magnitude 1, angle 90 degrees
(3) magnitude 1, angle 180 degrees