Maths Olympiad Prep

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, 2008

Algebra Difficulty 5.0 AIME Prove it JBMO

Problem:
Find all triples (x,y,z)(x, y, z) of real numbers that satisfy the system
{x+y+z=2008x2+y2+z2=602421x+1y+1z=12008 \left\{\begin{array}{l} x + y + z = 2008 \\ x^{2} + y^{2} + z^{2} = 6024^{2} \\ \frac{1}{x} + \frac{1}{y} + \frac{1}{z} = \frac{1}{2008} \end{array}\right.

Solution

Solution:
The last equation implies xyz=2008(xy+yz+zx)x y z = 2008(x y + y z + z x), therefore
xyz2008(xy+yz+zx)+20082(x+y+z)20083=0. x y z - 2008(x y + y z + z x) + 2008^{2}(x + y + z) - 2008^{3} = 0.
(x2008)(y2008)(z2008)=0(x - 2008)(y - 2008)(z - 2008) = 0.
Thus one of the variables equals 20082008. Let this be xx. Then the first equation implies y=zy = -z. From the second one it now follows that
2y2=6024220082=20082(91)=240162. 2 y^{2} = 6024^{2} - 2008^{2} = 2008^{2}(9 - 1) = 2 \cdot 4016^{2}.
Thus (x,y,z)(x, y, z) is the triple (2008,4016,4016)(2008, 4016, -4016) or any of its rearrangements.

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