Maths Olympiad Prep

Library / /260 of 377

Algebra Difficulty 5.3 AIME, harder Prove it United States

Problem:
Let aa, bb, and cc be complex numbers such that a=b=c=1|a|=|b|=|c|=1. If
a2bc+b2ca+c2ab=1 \frac{a^{2}}{b c}+\frac{b^{2}}{c a}+\frac{c^{2}}{a b}=1
as well, determine the product of all possible values of a+b+c|a+b+c|.

Solution

Solution:
Let s=a+b+cs=a+b+c. Then
s3=a3+b3+c3+3(a2b+ab2+b2c+bc2+c2a+ca2)+6abc=abc(a2bc+b2ca+c2ab+3(ab+ba+bc+cb+ca+ac)+6)=abc(1+(3(a+b+c)(1a+1b+1c)9)+6)=abc(3s(1a+1b+1c)2)=abc(3ssˉ2)( because sˉ=aˉ+bˉ+cˉ=1/a+1/b+1/c)=abc(3s22) \begin{aligned} s^{3} & =a^{3}+b^{3}+c^{3}+3\left(a^{2} b+a b^{2}+b^{2} c+b c^{2}+c^{2} a+c a^{2}\right)+6 a b c \\ & =a b c\left(\frac{a^{2}}{b c}+\frac{b^{2}}{c a}+\frac{c^{2}}{a b}+3\left(\frac{a}{b}+\frac{b}{a}+\frac{b}{c}+\frac{c}{b}+\frac{c}{a}+\frac{a}{c}\right)+6\right) \\ & =a b c\left(1+\left(3(a+b+c)\left(\frac{1}{a}+\frac{1}{b}+\frac{1}{c}\right)-9\right)+6\right) \\ & =a b c\left(3 s\left(\frac{1}{a}+\frac{1}{b}+\frac{1}{c}\right)-2\right) \\ & =a b c(3 s \bar{s}-2) \quad(\text{ because } \bar{s}=\bar{a}+\bar{b}+\bar{c}=1 / a+1 / b+1 / c) \\ & =a b c\left(3|s|^{2}-2\right) \end{aligned}
Taking absolute values, we find s3=3s22|s|^{3}=\left|3|s|^{2}-2\right|. It follows that s|s| must be a positive real root of x33x2+2=0x^{3}-3 x^{2}+2=0 or x3+3x22=0x^{3}+3 x^{2}-2=0. However, since the negative real roots of x33x2+2=0x^{3}-3 x^{2}+2=0 are exactly the additive inverses of the positive real roots of x33x2+2=0x^{3}-3 x^{2}+2=0, and all three roots of x33x2+2=0x^{3}-3 x^{2}+2=0 are real (x33x2+2=0(x^{3}-3 x^{2}+2=0 may be factored as (x1)(x22x2)=0(x-1)(x^{2}-2 x-2)=0, and the discriminant of x22x2x^{2}-2 x-2 is positive), the product of all possible values of s|s| is (2)(1)n(-2) \cdot(-1)^{n}, where nn denotes the number of negative real roots of x33x2+2=0x^{3}-3 x^{2}+2=0. By Descartes's Rule of Signs, we see that nn 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.