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Student Support Forum > General > > "Plotting non-linear values and solving"

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Original Message (ID '85425') By sk597653:
In Response To 'Re: Re: Re: Re: Re: Re: Re: Plotting non-linear...' --------- Yet another incarnation: c= 1.32 b = 1 + k c + k x/(2 s) y2 = (b - (Sqrt[b^2 - 2 k^2 c x/s]))/(2 k ) Therefore 1 + 1.32 k + (k x)/(2 s) - Sqrt[-((2.64 k^2 x)/s) + (1 + 1.32 k + (k x)/(2 s))^2])/(2 k) modified data to read- data = {{0.0, 0.0}, {0.79333, 0.4176}, {1.19, 0.5385}, {1.5867, 0.6374}, {1.9833, 0.7473}, {2.38, 0.8571}, {2.7767, 0.9231}, {3.1733, 0.9560}, {3.57, 0.9780}, {3.9667, 0.9780}, {4.3633, 0.9890}, {4.76, 1.0220}, {5.1567, 0.9890}, {5.5533, 1.0110}, {6.3467, 1.0000}, {7.5367, 1.0000}, {8.7267, 1.0110}, {10.313, 1.0000}} Hopefully this modification will correct for the magnitude of error in our resutlt. Now for: fit = FindFit[data, y2, {k, s}, x] And I get error: FindFit::nrlnum: The function value {0. +0. I,-0.497189+0. I,-0.661709+0. I,-0.807129+0. I,-0.966744+0. I,-1.12983+0. I,-1.2531+0. I,-1.34771+0. I,-1.43645+0. I,-1.50889+0. I,-1.5988+0. I,-1.7182+0. I,-1.78024+0. I,-1.90736+0. I,-2.14485+0. I,-2.66116+0. I,-3.53309+0. I,-7.8039+2.08105 I} is not a list of real numbers with dimensions {18} at {k,s} = {-0.0187691,0.134454}. >> {k -> -0.0187691, s -> 0.134454} Now, again with this exact data set, the k and s should be 2900000 and 0.84 respectively, that is what Sigmaplot gave. I do not have any of the information other than this from sigmaplot as this student is no longer here. Furthermore, I did calculate by hand with the equation and data given. When I used 0.79333 as x, and the k and s previously discussed, I calculated y to be 0.47222, not bad, right?! Again I tried with the last data point, x= 10.313 and got y to be 1.31999. What is going on?! Can you please try calculation as I did by hand. I don't think I made gross errors with my math as I reproduced it a couple of times and with different datapoints. So, for this data, and the given equation, a result of k=2900000 and s =0.84 does not seem far fetched! What is going on Mathematica? I know this should work. Any ideas with the newly modified formatting of the x-axis data? Samantha