Maths Olympiad Prep

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Algebra Difficulty 5.0 AIME Prove it United States

Problem:

Prove that there do not exist pairwise distinct complex numbers aa, bb, cc, and dd such that
a3bcd=b3cda=c3dab=d3abc. a^{3}-b c d=b^{3}-c d a=c^{3}-d a b=d^{3}-a b c .

Solutions — 2

Solution 1

Solution:

First suppose none of them are 00. Let the common value of the four expressions be kk, and let abcd=Pa b c d = P. Then for x{a,b,c,d}x \in \{a, b, c, d\},
x3Px=kx4kxP=0 x^{3} - \frac{P}{x} = k \Longrightarrow x^{4} - k x - P = 0
However, Vieta's tells us abcd=Pa b c d = -P, meaning P=PP = -P, so P=0P = 0, a contradiction.

Now if a=0a = 0, then bcd=b3=c3=d3-b c d = b^{3} = c^{3} = d^{3}. Then without loss of generality b=xb = x, c=xωc = x \omega, and d=xω2d = x \omega^{2}. But then bcd=x3x3-b c d = -x^{3} \neq x^{3}, a contradiction.

Thus, there do not exist distinct complex numbers satisfying the equation.

Solution 2

Solution:

Subtracting the first two equations and dividing by aba-b gives a2+b2+ab+cd=0a^{2} + b^{2} + a b + c d = 0. Similarly, c2+d2+ab+cd=0c^{2} + d^{2} + a b + c d = 0. So, a2+b2=c2+d2a^{2} + b^{2} = c^{2} + d^{2}. Similarly, a2+c2=b2+d2a^{2} + c^{2} = b^{2} + d^{2}. So, b2=c2b^{2} = c^{2}. Similarly, a2=b2=c2=d2a^{2} = b^{2} = c^{2} = d^{2}. Now by Pigeonhole, two of these 4 must be the same.

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