Curve fitting with a known coefficient

I want to fit set of data points (x,y) to the equation 'g*x+a*(x/248.7860)^3*c'. Here g and a are unknown coefficients and x is independent variable. c is coefficient which is an array of size same as x (c varies with x but it is not weight) c is known. How to do the fitting? I tried the following: ft = fittype( 'g*x+a*(x/248.7860)^3*c', 'independent', 'x','independent', 'c', 'dependent', 'y' ); But it is not working. Any other way to solve?

[Valentin Fazenda]

I have the same problem but with a complex equation :

PA=fittype('(Fz*(b1*Fz+b2))*sind(b0*atand((((b3*(Fz^2)+b4*Fz)*exp(-b5*Fz))/(b0*(Fz*(b1*Fz+b2))))*(x+(b9*Fz+b10))-((b6*Fz^2+b7*Fz+b8)*(1-b13*sind(x+(b9*Fz+b10))))*((((b3*(Fz^2)+b4*Fz)*exp(-b5*Fz))/(b0*(Fz*(b1*Fz+b2))))*(x+(b9*Fz+b10))-atand((((b3*(Fz^2)+b4*Fz)*exp(-b5*Fz))/(b0*(Fz*(b1*Fz+b2))))*(x+(b9*Fz+b10))))))+(b11*Fz+b12)','problem', 'Fz');

where Fz is known and is an (known) array of size same as x and i dont' know how write it so that Fz will not anymore be calaculated but used by the computer

Answer 1

[David Sanchez]

You can use the curve fitting tool. Type:

cftool

at the command window. Then, select " custom equation " as your target and type your equation on the right box.

[Mahesha MG]

I tried this. But it takes c as unknown coefficient and gives some value for it.

Answer 2

[Matt J]

Functions from the Curve Fitting Toolbox seem like overkill here, considering that you have a very simple, linear fitting to do:

 A=[x(:), (x(:)/248.7860).^3.*c(:)];

 p=A\y(:);

 g=p(1);
 a=p(2);