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I have three given points:

$\qquad $A$\qquad $(0; 5.9)
$\qquad $B$\qquad $(18; 5.56)
$\qquad $C$\qquad $(22.32; 0)

I don't know how I can create a function like the one in the image below with those points. Would be great if anybody could help me!

enter image description here

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2 Answers 2

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Physically motivated answer:

points = {{0, 5.9}, {18, 5.56}, {22.32, 0}};
pointsSharp = {{0, 5.9}, {22.31, 5.89}, {22.32, 0}};
nmf = NonlinearModelFit[points, 
   a (1/(1 + Exp[(x - b)/c]) - 1/2), {a, b, c}, x];
nmfSharp = NonlinearModelFit[pointsSharp, 
   a (1/(1 + Exp[(x - b)/c]) - 1/2), {a, b, c}, x];
blue = RGBColor[17.6/100, 41.6/100, 63.1/100];
red = RGBColor[86.3/100, 13.3/100, 19.6/100];
gr = Show[
 ListPlot[points, PlotStyle -> Directive[PointSize[Large], red]], 
 Plot[{nmfSharp[x], nmf[x]}, {x, 0, 30}, 
  PlotStyle -> {Directive[Thick, Dashed, Black], 
  Directive[Thick, blue]}],    
  AxesLabel -> {"V", "I"}, 
  AxesStyle -> Directive[AbsoluteThickness[1], 14, FontFamily -> "Helvetica"]
]

enter image description here

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points = {{0, 5.9}, {18, 5.56}, {22.32, 0}};

Graphics[{Red, PointSize[Medium], Point@points,
  Blue, BezierCurve[points, SplineDegree -> 1],
  Thick, Green, BezierCurve[points]}, Frame -> True, 
 AspectRatio -> 1/GoldenRatio]

Mathematica graphics

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