AlgebraDifficulty 5.3AIME, harderProve itUnited States
Problem: Let a, b, and c be complex numbers such that ∣a∣=∣b∣=∣c∣=1. If bca2+cab2+abc2=1 as well, determine the product of all possible values of ∣a+b+c∣.
Solution
Solution: Let s=a+b+c. Then s3=a3+b3+c3+3(a2b+ab2+b2c+bc2+c2a+ca2)+6abc=abc(bca2+cab2+abc2+3(ba+ab+cb+bc+ac+ca)+6)=abc(1+(3(a+b+c)(a1+b1+c1)−9)+6)=abc(3s(a1+b1+c1)−2)=abc(3ssˉ−2)( because sˉ=aˉ+bˉ+cˉ=1/a+1/b+1/c)=abc(3∣s∣2−2) Taking absolute values, we find ∣s∣3=3∣s∣2−2. It follows that ∣s∣ must be a positive real root of x3−3x2+2=0 or x3+3x2−2=0. However, since the negative real roots of x3−3x2+2=0 are exactly the additive inverses of the positive real roots of x3−3x2+2=0, and all three roots of x3−3x2+2=0 are real (x3−3x2+2=0 may be factored as (x−1)(x2−2x−2)=0, and the discriminant of x2−2x−2 is positive), the product of all possible values of ∣s∣ is (−2)⋅(−1)n, where n denotes the number of negative real roots of x3−3x2+2=0. By Descartes's Rule of Signs, we see that n is odd, so the answer is 2, as desired.
Want a route through all this instead of an archive? The track
puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.
Source: MathNet,
licensed CC-BY-4.0.
Statement reproduced verbatim; metadata (topic, difficulty) added by this project.