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Algebra Difficulty 4.9 AIME Prove it Estonia

Find all positive real-valued solutions to
{xy+1z=2013,yz+1x=2013,zx+1y=2013. \begin{cases} x - y + \frac{1}{z} = 2013, \\ y - z + \frac{1}{x} = 2013, \\ z - x + \frac{1}{y} = 2013. \end{cases}

Solutions — 2

Solution 1

Suppose w.l.o.g. that zxz \ge x and zyz \ge y. From the second equation 1x2013\frac{1}{x} \ge 2013,

therefore x12013x \le \frac{1}{2013}. From the third equation 1y2013\frac{1}{y} \le 2013, due to which y12013xy \ge \frac{1}{2013} \ge x.
But now from the first equation 1z2013\frac{1}{z} \ge 2013, therefore z12013z \le \frac{1}{2013}. Since we assumed that zy12013z \ge y \ge \frac{1}{2013}, the only possibility is z=y=12013z = y = \frac{1}{2013}, then also x=12013x = \frac{1}{2013}.

Solution 2

Adding up all the equations, we get 1x+1y+1z=32013\frac{1}{x} + \frac{1}{y} + \frac{1}{z} = 3 \cdot 2013. Multiplying the first equation by zz, the second one by xx and third one by yy and adding together we get 3=2013(x+y+z)3 = 2013(x + y + z). Therefore the arithmetic and harmonic mean of x,y,zx,y,z are both equal to 12013\frac{1}{2013}. Consequently x=y=z=12013x = y = z = \frac{1}{2013}.

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