# Find Unknown coefficient using Contour plot

I have I really complicated equation, but for simplicity lets assume my function is F(x,y,z)= ax+by+c*z, where a, b and c are some unknown constants.

From experiments we know that at:

x=3, y=2, z=1 then F=10

x=2, y=1, z=0 then F=4

x=1, y=0, z=0 then F=1

So, is there any way to plot F(x,y,z) as contour plot and then fit my data above in order to extract the exact unknown coefficient?

By the way the example answer is a=1, b=2 and c=3.

• for the example posted, you can use {a, b, c} = LinearSolve[data[[All, ;; 3]], data[[All, -1]]] or {a, b, c} = LeastSquares[data[[All, ;; 3]], data[[All, -1]]] – kglr Dec 11 '19 at 23:15

Clear["Global*"]

f[x_, y_, z_] = a*x + b*y + c*z;


Your data is the form {x, y, z, f}

data = {{3, 2, 1, 10}, {2, 1, 0, 4}, {1, 0, 0, 1}};


Each set of the data generates one equation

eqns = f @@ Most[#] == Last@# & /@ data

(* {3 a + 2 b + c == 10, 2 a + b == 4, a == 1} *)


There are three equations and three unknown coefficients; consequently, the system can be solved

sol = Solve[eqns, {a, b, c}][[1]]

(* {a -> 1, b -> 2, c -> 3} *)


The form of the function is

f[x, y, z] /. sol

(* x + 2 y + 3 z *)

• Since my data are experimental (more than three points) and with complicated relation I was unable to solve it using system of equation. That's why if this problem can be solved using contour plot or in any other graphical technique ? – Love Eva Nov 12 '19 at 2:31
• If you can make a plot you must have a model. Is there something that prevents you from using NonlinearModelFit` to fit your data to that model? – Bob Hanlon Nov 12 '19 at 3:57