answer 3:1 - least squares fit of Y = [X]/(K+[X])

The use of least squares minimization is the soundest statistical way of analyzing the experimental data. You use the following equation to calculate the fraction of liganded sites:
Y = [X] / (K + [X])
The following results are obtained:

The equilibrium constant obtained is Kd = 1 mM (for the dissociation reaction: PX <==> P + X).

Do the fit and the residuals look good?
1) Yes
2) No

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