Fit[]: how to obtain the parameter values?

*To*: mathgroup at smc.vnet.net*Subject*: [mg31825] Fit[]: how to obtain the parameter values?*From*: Peter Marzlin <peter.marzlin at uni-konstanz.de>*Date*: Wed, 5 Dec 2001 06:51:50 -0500 (EST)*Organization*: University of Constance, Germany*Sender*: owner-wri-mathgroup at wolfram.com

Hi all, I am trying to use Fit to get a linear least square fit with some complicated functions. In principle this works fine, but I cannot get the parameter values. For instance, using the two functions f1[x] = 2 and f2[x] = 3x I would like to get the parameters a1 and a2 in the fit function f[x] = a1 f1[x] + a2 f2[x] for the set of data data = {{1, 2}, {2, 2.5}, {3, 3}} Using the command Fit[data, {f1[x], f2[x]}, x] I get the answer 1.5 + 0.5 x This is correct but not what I want. I need a1 and a2, but they cannot easliy be obtained because Fit merges the prefactors in the definition of f1 and f2 with the value of the parameters a1 and a2. Of course, in this simple example I can get a1 and a2 by hand, but I am actually interested in a much more involved problem where extracting the parameter values from the Fit output is a rather tedious if not practically impossible task. Any help would be greatly appreciated. Peter