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If ω is a complex cube root of unity, show that (2 + ω + ω2)3 − (1 − 3ω + ω2)3 = 65

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Question

If ω is a complex cube root of unity, show that (2 + ω + ω2)3 − (1 − 3ω + ω2)3 = 65

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Solution

ω is the complex cube root of unity

∴ ω3 = 1 and 1 + ω + ω2 = 0

Also, 1 + ω2 = − ω, 1 + ω = − ω2 and ω + ω2 = − 1

L.H.S. = (2 + ω + ω2)3 − (1 − 3ω + ω2)3

= [2 + (ω + ω2)]3 − [− 3ω + (1 + ω2)]3

= (2 − 1)3 − (− 3ω − ω)3

= 13 − (− 4ω)3

= 1 + 64ω3

= 1 + 64(1)

= 65

= R.H.S.

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Chapter 1: Complex Numbers - Exercise 1.4 [Page 20]

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