Proving Isometry on Metric: How to Do It

[jamiemmt]

How do I prove every isometry from R->R is of the form f(x)=a+-x , regardless of the metric?

I know it has to do with considering d_1(0,x_1)=d_2(0, f(x_1)), but beyond that I am lost.

 

[HallsofIvy]

Start by writing out the definition of "isometry"!