I think you don't need to define a user-defined function for:
because Origin has a built-in function Rational4 in Rational category:
In the fitting process of NLFit, you can fix the value of c (of Rational4) to 0
(by turning ON the "Fixed" check box in the Parameter tab, and double-click
the value, and enter 0).
By the way, for defining a user-defined function, maybe the following
tutorials may be helpul:
As your Origin version is 8.0, it doesn't have more convenient
"Fitting Function Builder" (available in 8.5, or later), and
you need to skip the section in the (Text) tutorial.
Hope this helps.