# Solve does not produce a result for a system of equations.

Posted 3 months ago
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 I have defined a function as: Clear["Global*"]; Clear[Derivative]; D1[Q_, T_, D0_, k_] = D0*Exp[-Q/(k*T)]; This code produces a result when trying to find the parameters D0 and Q given the value of D1 at two values of T. k is a fixed parameter. EQ1 = D1[Q, 650 + 273.15, D0, 9.62*10^-5] == 5.5*10^-16; Temp = Solve[{EQ1}, {Q}]; EQ2 = D1[Q /. Temp[[1]], 900 + 273.15, D0, 9.62*10^-5] == 1.3*10^-13; SolD0 = Solve[EQ2] EQ3 = D1[Q, 650 + 273.15, D0 /. SolD0[[1]], 9.62*10^-5] == 5.5*10^-16; Solve[EQ3] Output: {{D0 -> 0.0000756184}} and {{Q -> 2.27762}} Trying to solve the equations as a set produces no result. Note I tried both Solve and NSolve. Solve[{D1[Q, 650 + 273.15, D0, 9.62*10^-5] == 5.5*10^-16, D1[Q, 900 + 273.15, D0, 9.62*10^-5] == 1.3*10^-13}, {Q, D0}] NSolve[{D1[Q, 650 + 273.15, D0, 9.62*10^-5] == 5.5*10^-16, D1[Q, 900 + 273.15, D0, 9.62*10^-5] == 1.3*10^-13}, {D0, Q}] Output: {{D0 -> 0.}} and {{D0 -> 0.}} D0=0 is not a solution to these equations.I do get the warning Solve: Equations may not give solutions to all "solve" variables. from both the Solve and NSolve functions.Any ideas what is going on and what I need to do to Solve the equations as a simultaneous set?
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Posted 3 months ago
 Note I misstyped the example that was giving me an issue. The equations are indeed different, and the call to Solve should be: Solve[{D1[Q, 650 + 273.15, D0, 9.62*10^-5] == 5.5*10^-16, D1[Q, 900 + 273.15, D0, 9.62*10^-5] == 1.3*10^-13}, {Q, D0}] This results in the same issue.
 I found a work around. Solve[{Log[D1[Q, 900 + 273.15, D0, 9.62*10^-5]] == Log[1.3*10^-13], Log[D1[Q, 650 + 273.15, D0, 9.62*10^-5]] == Log[5.5*10^-16]}] Output is correct: {{D0 -> 0.0000756184, Q -> 2.27762}} `I would really like an explanation of why this works if anyone knows.