Maths Olympiad Prep

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, 2010

Algebra Difficulty 5.3 AIME, harder Prove it Austria

Determine all triples of real numbers (x,y,z)(x, y, z), such that the equation
4x4x2(4y4+4z41)2xyz+y8+2y4z4+y2z2+z8=0 4x^4 - x^2 (4y^4 + 4z^4 - 1) - 2xyz + y^8 + 2y^4z^4 + y^2z^2 + z^8 = 0
holds.

Solution

We first note that
4x4x2(4y4+4z41)2xyz+y8+2y4z4+y2z2+z8=(4x4+y8+z84x2y44x2z4+2y4z4)+(x22xyz+y2z2)=(2x2y4z4)2+(xyz)2. \begin{aligned} & 4x^4 - x^2(4y^4 + 4z^4 - 1) - 2xyz + y^8 + 2y^4z^4 + y^2z^2 + z^8 \\ & = (4x^4 + y^8 + z^8 - 4x^2y^4 - 4x^2z^4 + 2y^4z^4) + (x^2 - 2xyz + y^2z^2) \\ & = (2x^2 - y^4 - z^4)^2 + (x - yz)^2. \end{aligned}
The given equation is therefore equivalent to
(2x2y4z4)2+(xyz)2=0. (2x^2 - y^4 - z^4)^2 + (x - yz)^2 = 0.
It therefore follows that both x=yzx = yz and 2x2y4z4=02x^2 - y^4 - z^4 = 0 must hold.
Substituting x=yzx = yz in the second of these equations, we obtain 2y2z2y4z4=02y^2z^2 - y^4 - z^4 = 0, which is equivalent to (y2z2)2=0-(y^2 - z^2)^2 = 0. We see that z=±yz = \pm y must hold. For z=y=tz = y = t, we obtain x=t2x = t^2, and for z=y=t-z = y = t, we obtain x=t2x = -t^2. It therefore follows that the set of all solutions is
{(t2,t,t)tR}{(t2,t,t)tR}. \{ (t^2, t, t) \mid t \in \mathbb{R} \} \cup \{ (-t^2, t, -t) \mid t \in \mathbb{R} \}.

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