You have not defined any variable named x.
It is like you call this:
sin(x)
but have not specified x anywhere. MATLAB does not invent values to use, it uses the values that you have defined, or else it throws an error. In this case, because you have not defined any variable x, it throws an error.
But really you should avoid using feval and use function handles instead. These are more robust, are much neater, and allow you to use the standard editor code hinting and debugging tools.
