How to fit piecewise function while bound is uncertain?

Dear ROOT expert:

I want to fit a line like this:

y = par[0]* x **2 (x<par[2])
y = par[1] *x **3 (x >=par[2])

how can I apply this in root fit function?

Thanks,Gang

The solution is found.

Thanks,Gang

This is an old question, but does anyone know how to do this?

{
  TF1 *f = new TF1("f", "((x < [2]) ? [0] : [1] * x) * x * x", 1., 7.);
  f->SetParameters(3., 4., 5.);
  f->Draw();
  gPad->SetLogy();
}

Thanks! That worked perfectly!