Calculating resistors to go with a thermistor
Lucky you, I just had to figure this out for a similar part.  Can we attach a spreadsheet here?  If not, just assume your Rc is in cell A2 and Rh is in cell B2.  Due to the quadratic, there's two solutions for R2:

=(-19*A2-19*B2+(19*(19*A2^2+19*B2^2-38*A2*B2-4*B2^2*A2+4*B2*A2^2))^0.5)/2/(B2-A2+19)

=(-19*A2-19*B2-(19*(19*A2^2+19*B2^2-38*A2*B2-4*B2^2*A2+4*B2*A2^2))^0.5)/2/(B2-A2+19)

(Those can be copy/pasted directly into Excel.)

For each of those, you can calculate R1 (B5 being the cell with an R2 solution):

=5-B5*B$2/(B5+B$2)

Hopefully, one of the solutions will have both resistances positive.  If anyone has an idea how to attach a spreadsheet here, I'll do that.