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Screensize: 640 x 480 800 x 600 1024 x 768 1280 x 1024 UserName: Password: Anti-Spam Code: Format Mode: Basic  Help  Prompt  Format: Font Andale Mono Arial Arial Black Book Antiqua Century Gothic Comic Sans MS Courier New Georgia Impact Lucida Console Script MT Bold Stencil Tahoma Times New Roman Trebuchet MS Verdana   Size 1 2 3 4 5 6   Color Black Red Yellow Pink Green Orange Purple Blue Beige Brown Teal Navy Maroon LimeGreen Message: * HTML is OFF * Forum Code is ON Smilies [quote][i]Originally posted by sevny[/i] [br]Origin Ver. and Service Release (Select Help-->About Origin): Origin 9.0 Operating System:Windows OK, here is the problem I am running into, simply, I am trying nonlinear curve fitting using multiple equations with intermediate variables. It worked well for the simple ones, but when quadratic or higher order equations are present, I have the problem of how to select the correct root out of the multiple roots. The equations are as follows. In the following equations, x is independent, y is dependent; H and Ka are parameters; L0,P0,R0,V0,Vi and fi are constants; L1, L2, P1 and P2 are intermediate variables, L1 or L2 are greater than zero, smaller than L0; P1 and P2 are greater than zero, smaller than P0. When solve the equations, two L1 and two L2 will be generated from the quadratic equation, so the difficulty is how to realize root selection here. Thank you very much for your time. Any advice or comment are appreciated. L1=L0*(1-R0/(R0+x))/(1+Ka*P1); P1=P0*R0/((R0+x)*(1+Ka*L1)); L2=L0*(1-R0/((R0+x)*fi))/(1+Ka*P2); P2=P0*R0/((R0+x)*fi*(1+Ka*L1)); y=V0*H*Ka*(P1*L1-P2*L2*fi)/(Vi*L0); [/quote]   Check here to include your profile signature. Check here to subscribe to this topic.