I had to assume some things about calling your function, since you did not provide enough information to actually try to run your code. The following function (that calls your function) worked just fine:
function answerTest()
global c_x beta
c_x = 1;
beta = 1;
functionToMinimise(1,rand(1,3),1)
end
function f = functionToMinimise(x,Input,Actual_Output)
% f = x^2 %f and x are dummy variables
global c_x beta
Fitted_Curve = (1/(sqrt(x)*sqrt(beta)))*(c_x-asinh(sinh(c_x)*exp(x*Input*2*pi)));
f = Fitted_Curve;
end
