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2002 AMC 12A Problem 24

Problem 24 of 25HarderAlgebra

Find the number of ordered pairs of real numbers (a,b)(a, b) such that (a+bi)2002=a−bi.(a + bi)^{2002} = a - bi.

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Solution

Let z=a+bi.z = a + bi. The equation is z2002=z‾.z^{2002} = \overline{z}. Taking magnitudes, ∣z∣2002=∣z∣,|z|^{2002} = |z|, so ∣z∣(∣z∣2001−1)=0,|z|\big(|z|^{2001} - 1\big) = 0, giving ∣z∣=0|z| = 0 or ∣z∣=1.|z| = 1. If ∣z∣=0,|z| = 0, then (a,b)=(0,0),(a, b) = (0, 0), one solution. If ∣z∣=1,|z| = 1, then z‾=1z,\overline{z} = \dfrac1z, so z2002=1z,z^{2002} = \dfrac1z, i.e. z2003=1,z^{2003} = 1, which has 20032003 distinct roots. Altogether there are 1+2003=20041 + 2003 = 2004 ordered pairs. Thus, the correct answer is E.
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