Distance function is in fact a metric I know I should be able to show this, but for some reason I am having trouble. I need to show that $$d(x,y) = \frac{|x-y|}{1+|x-y|}$$ is a metric on $\Bbb R$ where $|*|$ is the absolute value metric. I am getting confused trying to show that the triangle inequality holds for this function. My friend also said that he proved that this distance function defines a metric even if you replace $|*|$ with any other metric. So I'd like to try and show both, but I cannot even get the specific case down first. Please help.

Let $f(x)=x/(1+x)$, so  $f$ is continuous on $[0,\infty)$, maps $[0,\infty)$ into itself, is non-decreasing  since$f(x)=1-1/(x+1)$, $f(0)=0$ and subadditive, since $f''=-2/(1+x)^3<0$ for $x>0$.
It's easy to show that if a function satisfies the above properties, for  any metric $d$ then $f\circ d$ is a metric. 
To verify that $f''<0$ give us subbaditivity. 
$f'$ is decreasing so we can use the following result click
