Error on Function integral

Dear Rooters,
I’ve a simple but nasty problem. I’ve a function (a broken powe-law) which I’m fitting to graph. I need to compute the integral of the best-fit function and its error.
How can I do that ? Is there any post about it? Any function which already implements it?

Suggestions always welcome.

see example at $ROOTSYS/tutorials/fit/ErrorIntegral.C