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2023 AMC 12A Problem 14

Problem 14 of 25IntermediateAlgebra

How many complex numbers satisfy the equation z5=z,z^5=\overline{z}, where z\overline{z} is the conjugate of the complex number z?z?

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Solution

Taking magnitudes gives z5=z,|z|^5=|z|, so z=0|z|=0 or z=1.|z|=1. The value z=0z=0 works, giving one solution. If z=1,|z|=1, multiply the equation by zz to get z6=zz=z2=1.z^6=z\overline{z}=|z|^2=1. This has 66 distinct roots, all of modulus 1.1. Altogether there are 1+6=71+6=7 solutions. Thus, the correct answer is E.

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Concepts: complex number · roots of unity

Problem text and solution from the LIVE past-contest archive. See also the AoPS wiki page for community solutions.