Undefined function or variable 'x'. Why is there an error?

%Undefined function or variable 'x'.
Error in linearinterp (line 2)
  f=(exp(-x/2).*sin(x.^2+8));%

function linearinterp
  %Error
  f=(exp(-x/2).*sin(x.^2+8));

    points = refine(0, 7, 0.6);

    plot(points, f(points), 'r+');

    hold on

    plot(points, f(points));

    hold off

end

function points = refine(x1, x2, tol)

    xm = (x1+x2)/2;

if abs((f(x1)+f(x2))/2-f(xm))<tol

    points=[x1,x2];

else

    left=refine(x1, xm, tol);

    right=refine(xm, x2, tol);

    points=[left, right(2:length(right))];

end

end

[Ehi Eromosele]

I mean f(x)=(exp(-x/2).*sin(x.^2+8));

Answer 1

[Walter Roberson]

f = @(x) (exp(-x/2).*sin(x.^2+8));