$f(x) = \frac{ab}{a+b} + \frac{ac}{a+c} + \frac{ad}{a+d} + \frac{ae}{a+e}+
\frac{bc}{b+c} + \frac{bd}{b+d} + \frac{be}{b+e}+
\frac{cd}{c+d} + \frac{ce}{c+e}+
\frac{de}{d+e}
$