2024 AIME II Problem 13

Below is the professionally curated solution for Problem 13 of the 2024 AIME II, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2024 AIME II solutions, or check the answer key.

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Concepts:roots of unitycomplex numberpolynomial

Difficulty rating: 3060

13.

Let ω1\omega \neq 1 be a 1313th root of unity. Find the remainder when k=012(22ωk+ω2k)\prod_{k=0}^{12} \left(2 - 2\omega^k + \omega^{2k}\right) is divided by 1000.1000.

Solution:

Since 22x+x2=(x1)2+1=(x(1+i))(x(1i)),2 - 2x + x^2 = (x - 1)^2 + 1 = (x - (1+i))(x - (1-i)), each factor of the product splits, and as kk runs from 00 to 12,12, ωk\omega^k runs over all 1313th roots of unity. Because k(xωk)=x131,\prod_k (x - \omega^k) = x^{13} - 1, for any α\alpha we get k(ωkα)=(1)13(α131)=1α13.\prod_k (\omega^k - \alpha) = (-1)^{13}(\alpha^{13} - 1) = 1 - \alpha^{13}. Hence the product equals (1(1+i)13)(1(1i)13).\left(1 - (1+i)^{13}\right)\left(1 - (1-i)^{13}\right).

Since (1+i)2=2i,(1+i)^2 = 2i, we get (1+i)13=(1+i)(2i)6=64(1+i)=6464i,(1+i)^{13} = (1+i)(2i)^6 = -64(1 + i) = -64 - 64i, and by conjugation (1i)13=64+64i.(1-i)^{13} = -64 + 64i. So the product is (65+64i)(6564i)=652+642=4225+4096=8321,(65 + 64i)(65 - 64i) = 65^2 + 64^2 = 4225 + 4096 = 8321, whose remainder upon division by 10001000 is 321.321.

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