How do you find the cube root of z = 1 + i?

Firstly we express z in polar form:
 
z = Reiθ
 
where |z| = (Re2 + Im2)0.5 = (12 + 12)0.5 = 20.5
 
θ = arg z = tan-1(Im/Re) = tan-1(1/1) = π/4
 
Therefore z = (20.5)eiπ/4
 
We can add on any multiple of 2π to the argument of z without affecting the value of the complex number:
 
z = (20.5)ei(π/4 + 2πn)
 
where n is an integer
 
We then take cube roots of both sides (not forgetting to cube root the modulus R as well as the exponent):
 
z1/3 = (21/6)ei(π/12 + 2πn/3) = (21/6)ei(π + 8πn)/12
 
Because we are calculating the cube root, we expect three solutions. To find these three roots, we substitute in three consecutive integers into n. We will choose n = 0, 1, 2.
 
Solution 1 (with n=0): z1/3 = (21/6)ei(π/12)
Solution 2 (with n=1): z1/3 = (21/6)ei(3π/4)
Solution 3 (with n=2): z1/3 = (21/6)ei(17π/12)
 
We can convert these back into Cartesian form using:
 
z = R
(cosθ + i sinθ)
 
We find that:
 
Solution 1: z1/3 =(21/6)
(cos(π/12) + i sin(π/12)) = 1.08 + 0.291i
Solution 2: z1/3 = (21/6)
(cos(3π/4) + i sin(3π/4)) = -0.794 +0.794i
Solution 3: z1/3 = (21/6)*(cos(17π/12) + i sin(17π/12)) = -0.291-1.084i

AE
Answered by Aldwyn E. Further Mathematics tutor

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