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I have these functions:

f[dA_,dB_] :=(3.2 (0.389 +dA (-2 (0.175 + 0.012 dB) + 0.078 (-1 + dB) +0.012 (-1 + dB)^2) + dA^2 (0.214 - 0.027 dB) - 0.039 (-1 + dA) (-1 + dB)^2 + (-0.027 + 0.175 (-2 + dB)) dB) + (-1 + dA) (-1 + dB) (2.04 (-1 + dA) (-1 + dB) + (-1 + dA dB) Vin))/((-1 + dA) (-1 + dB) (-1 + dA dB) Vin)
g[dA_,dB_] :=-3.2 (0.214/(1 - dA) + (0.187 dA)/(1 - dA)^2) + ((1 - (1.02 (1 - dA))/Vin) Vin)/(1 - dA)
h[dA_,dB_] :=-3.2 (0.214/(1 - dB) + (0.187 dB)/(1 - dB)^2) + (dB (1 - (1.02 (1 - dB))/(dB Vin)) Vin)/(1 - dB)

I would like to find the maximum of f, with parameter Vin. Also, the following constraints must be met: 

0<dA<1 && 0<dB<1 && 0<Vin<=625 && g+h=625

I have been trying to use the function FindMaximum, but I think I do something wrong and I don't have any solution. 

FindMaximum[{f, 0 < dA < 1 && 0 < dB < 1 && 0 < Vin <= 625 && g + h=625},{dA, dB}]

Could somebody help me understand what is my mistake or suggest another way to solve my problem? I have been also trying to use Partial Differential Equations, but I didn't have results also there.

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You need to give numeric values to Vin.

This:

max = Table[{dA, dB} /. 
   FindMaximum[{f[dA, dB], 
      0 < dA < 1 && 0 < dB < 1 && g[dA, dB] + h[dA, dB] == 625},
        {{dA, 1/2}, {dB, 1/2}}, Method -> "InteriorPoint"][[2]], {Vin, 1, 625}]

or better this:

max = Table[
   FindArgMax[{f[dA, dB], 
     0 < dA < 1 && 0 < dB < 1 && g[dA, dB] + h[dA, dB] == 625},
      {{dA, 1/2}, {dB, 1/2}}, Method -> "InteriorPoint"], {Vin, 1, 625}];

shows that the maximum is along the line dA = dB for all Vin:

ListPlot[max, Frame -> True, PlotStyle -> Black, AspectRatio -> 1, 
 PlotRange -> {{0, 1}, {0, 1}}, FrameLabel -> {"dA", "dB"}]

enter image description here

To verify:

Manipulate[
 Plot3D[f[dA, dB] /. Vin -> v0, {dA, 0, 1}, {dB, 0, 1}],
  {{v0, 100}, 1, 625, Appearance -> {"Open"}}]

enter image description here

c = max[[100]]

{0.731275, 0.731275}

f @@ c /. Vin -> 100

0.970111

(g @@ c + h @@ c) /. Vin -> 100

625.

f[0.5, 0.5] /. Vin -> 100

0.976091

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  • $\begingroup$ Thank you very much for the analytical and quick answer. Indeed, I am expecting dA=dB for all Vin. However, is there a way to have the analytical solution, e.g. dA=function(Vin), dB=function(Vin)? $\endgroup$ – harazogo Jan 14 '18 at 12:10
  • $\begingroup$ Maximize doesn't work for me. Maybe you could use an implementation of the Lagrange multipliers. $\endgroup$ – corey979 Jan 14 '18 at 12:24

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