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Concurrent Curve Fitting... II

First of all, thank you for your numerous replies... I'm still working my 
way through them all...

Secondly, there was some confusion as to whether the xn and yn parameters
were known already or not... they are not: m and b are unknown and common to
both sets, x1 and y1 are unknown parameters for set 1, and x2 and y2 are
unknown parameters for set 2.
Also, this particular example can be reduced to the unknowns m (common) and
b1=b+y1-m*x2 (set 1) and b2=b+y2-m*x2 (set 2); I just left it in the
expanded form for clarity.

Thanks again,

original message:

Can anyone help me?

Here's what I want to do:

I have two sets of data which are related in such a way that, when plotted,
they should have the same shape (or I want to force the curve fits to have
the same shape), but at a different location.
So, for example (and simplicity} suppose I have two sets of data which can
be fit by lines, y=mx+b. I want to fit data set 1 to (y-y1)=m(x-x1)+b, and
data set 2 to (y-y2)=m(x-x2)+b, where m and b are common and xn and yn are
specific to the data set.
This is a fairly easy to do indirectly, even by hand, but is there a way to
get Mathematica to do this directly, i.e., finding the two curve fits
concurrently? Obviously, I'm trying to do this for much more complex models.

Any input would be appreciated.


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