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xData={1000,500,250,160,125,80,40,20,10,5,0};​​yData={0.03505,0.08309,0.10673,0.15438,0.22056,0.33907,0.48123,0.64192,0.76772,0.86004,1};​​KD1=1;​​KD2=6;​​LST=1;​​LT=xData;​​RT=3;​​Fsb=yData;​​a=KD1+KD2+LST+LT-RT;​​b=((LT-RT)*KD1)+((LST-RT)*KD2)+(KD1*KD2);​​c=-KD1*KD2*RT;​​ϕ=ArcCos[((-2a^3)+(9*a*b)-27c)/2Sqrt[(a^2-3b^3)]];​​​​model=(2Sqrt[a^2-3*b]*Cos[ϕ/3]-a)/(3*KD1+(2Sqrt[a^2-3b]*Cos[ϕ/3]-a));​​​​fit=FindFit[yData,model,KD2,xData]​​