15. $y=\frac{|x|-2}{|x|}$.

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15. $y=\frac{|x|-2}{|x|}$.
15. (Fig. 15) The root of the submodular expression is 0.

For $x>0, \quad y=1-\frac{2}{x}$.

For $x<0, \quad y=1+\frac{2}{x}$.