Maths Olympiad Prep

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Algebra Difficulty 4.8 AIME Prove it United States

Problem:

Complex number ω\omega satisfies ω5=2\omega^{5}=2. Find the sum of all possible values of
ω4+ω3+ω2+ω+1 \omega^{4}+\omega^{3}+\omega^{2}+\omega+1

Solution

Solution:

The value of ω4+ω3+ω2+ω+1=ω51ω1=1ω1\omega^{4}+\omega^{3}+\omega^{2}+\omega+1=\frac{\omega^{5}-1}{\omega-1}=\frac{1}{\omega-1}. The sum of these values is therefore the sum of 1ω1\frac{1}{\omega-1} over the five roots ω\omega. Substituting z=ω1z=\omega-1, we have that (z+1)5=2(z+1)^{5}=2, so z5+5z4+10z3+10z2+5z1=0z^{5}+5 z^{4}+10 z^{3}+10 z^{2}+5 z-1=0. The sum of the reciprocals of the roots of this equation is 51=5-\frac{5}{-1}=5 by Vieta's.

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