PRACTICE PROBLEMS ON FINDING ROOTS OF A COMPLEX NUMBER

Find the cube roots of a complex number

1)  2(cos 2π + i sin 2π)

2)  2(cos π/4 + i sin π/4)

3)  3(cos 4π/3 + i sin 4π/3)

4)  27(cos 11π/6 + i sin 11π/6)

5)  -2 + 2i

Solution

Answers :

1)  z0 3√2(cis (2π/3))

z1 3√2(cis (8π/3))

z2 3√2(cis (10π/3))

2)  z0  =  3√2(cis (π/12))

z1  =  3√2(cis (9π/12))

z2  =  3√2(cis (17π/12))

3)  z0  =  3√3(cis (4π/9))

z1  =  3√3(cis (10π/9))

z2  =  3√2[cis (16π/9))

4)  z0  =  3(cis (11π/18))

z1  =  3(cis (23π/18))

z2  =  3(cos (35π/18))

5)  z0  =  6√8(cis (3π/12))

z1  =  6√8(cos (11π/12))

z2  =  6√8(cos (19π/12))

Find the nth roots of a complex number

1)  1 + i,   n  =  4

2)  1 - i,   n  =  6

3)  2 + 2i,   n  =  3

4)  -2 + 2i,   n  =  4

Solution

Answers :

1)  z0  =  8√2(cis (π/16))

z1  =  8√2(cis (9π/16))

z2  =  8√2(cis (17π/16))

z3  =  8√2(cis (25π/16))

2)  z0  =  12√2(cis (π/24))

z1  =  8√2(cis (7π/24))

z2  =  12√2(cis (5π/8))

z3  =  12√2(cis (23π/24))

z4  =  12√2(cis (31π/24))

z5  =  12√2(cis (39π/24))

3)  z0  =  6√8(cis (π/12))

z1  =  6√8(cis (3π/4))

z2  =  6√8(cis (17π/12))

4)  z0  =  8√8(cis (3π/16))

z1  =  8√8(cis (11π/16))

z2  =  8√8(cis (19π/16))

z3  =  8√8(cis (27π/16))

Find the indicated power of a complex number

1)  (cos π/4 + i sin π/4)3

2)  [3(cos 3π/2 + i sin 3π/2)]5

3)  [2(cos 3π/4 + i sin 3π/4)]3

4)  (1 + i)5

5)  (1 - √3i)3

Solution

Answers : 

1)  z3  =  -√2/2 + i √2/2

2)  z5  =  243i

3)  z3  =  4√2 + i 4√2

4)  z5  =  -4 - 4i

5)  z3  =  -8

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